Grazie per le informazioni e i suggerimenti, Gioxx.
Comincio col dire che ho provato il Safe Mode, ma il problema non sparisce. 
Per quanto riguarda i componenti aggiuntivi, ecco cosa mi fornisce il comando config (qui sotto è brutalmente incollato, ma mi pare difficile da leggere. Ho fatto un PDF, ma non capisco come fare per allegarlo)
Giuseppe
PS: quale sarebbe il forum adatto, nel caso si trattasse di un problema del sistema operativo?
Informazioni di base
Nome 	Firefox
Versione 	56.0.2
ID build 	20171024165158
Cronologia aggiornamenti 	
Canale di aggiornamento 	release
User agent 	Mozilla/5.0 (Macintosh; Intel Mac OS X 10.12; rv:56.0) Gecko/20100101 Firefox/56.0
SO 	Darwin 16.7.0
Cartella del profilo 	
Plugin attivi 	about:plugins
Configurazione build 	about:buildconfig
Utilizzo memoria 	about:memory
Prestazioni 	about:performance
Service worker registrati 	about:serviceworkers
Finestre multiprocesso 	2/2 (Attivato automaticamente)
Processi contenuti web 	1/1
Stylo 	false (disattivato per impostazione predefinita)
Chiave Google 	Disponibile
Chiave per il servizio Mozilla Location 	Disponibile
Modalità provvisoria 	false
Profili 	about:profiles
Segnalazioni di arresto anomalo degli ultimi 3 giorni
ID segnalazione 	Inviata
Tutte le segnalazioni
Caratteristiche di Firefox
Nome 	Versione 	ID
Activity Stream	0.0.0	activity-stream@mozilla.org
Application Update Service Helper	2.0	aushelper@mozilla.org
Click-to-Play staged rollout	1.4	clicktoplay-rollout@mozilla.org
Firefox Screenshots	10.12.0	screenshots@mozilla.org
Follow-on Search Telemetry	0.9.3	followonsearch@mozilla.com
Form Autofill	1.0	formautofill@mozilla.org
Multi-process staged rollout	3.00	e10srollout@mozilla.org
Photon onboarding	0.1	onboarding@mozilla.org
Pocket	1.0.5	firefox@getpocket.com
Shield Recipe Client	65.1	shield-recipe-client@mozilla.org
Web Compat	1.1	webcompat@mozilla.org
Estensioni
Nome 	Versione 	Attiva 	ID
Safe Browsing Version 4 (temporary add-on)	1.0.0	true	sbv4-gradual-rollout@mozilla.com
DivX Plus Web Player HTML5 <video>	2.1.2.182	false	{23fcfd51-4958-4f00-80a3-ae97e717ed8b}
Grafica
Caratteristiche
Composizione	OpenGL
Panoramica/zoom asincroni (APZ)	input rotella attivo; trascinamento barre di scorrimento attivo
Informazioni driver WSI WebGL 1	CGL
Renderer driver WebGL 1	NVIDIA Corporation -- NVIDIA GeForce GT 330M OpenGL Engine
Versione driver WebGL 1	2.1 NVIDIA-10.2.37 310.90.10.05b54
Estensioni driver WebGL 1	GL_ARB_color_buffer_float GL_ARB_depth_buffer_float GL_ARB_depth_clamp GL_ARB_depth_texture GL_ARB_draw_buffers GL_ARB_draw_elements_base_vertex GL_ARB_draw_instanced GL_ARB_fragment_program GL_ARB_fragment_program_shadow GL_ARB_fragment_shader GL_ARB_framebuffer_object GL_ARB_framebuffer_sRGB GL_ARB_half_float_pixel GL_ARB_half_float_vertex GL_ARB_imaging GL_ARB_instanced_arrays GL_ARB_multisample GL_ARB_multitexture GL_ARB_occlusion_query GL_ARB_pixel_buffer_object GL_ARB_point_parameters GL_ARB_point_sprite GL_ARB_provoking_vertex GL_ARB_seamless_cube_map GL_ARB_shader_objects GL_ARB_shader_texture_lod GL_ARB_shading_language_100 GL_ARB_shadow GL_ARB_sync GL_ARB_texture_border_clamp GL_ARB_texture_compression GL_ARB_texture_compression_rgtc GL_ARB_texture_cube_map GL_ARB_texture_env_add GL_ARB_texture_env_combine GL_ARB_texture_env_crossbar GL_ARB_texture_env_dot3 GL_ARB_texture_float GL_ARB_texture_mirrored_repeat GL_ARB_texture_non_power_of_two GL_ARB_texture_rectangle GL_ARB_texture_rg GL_ARB_transpose_matrix GL_ARB_vertex_array_bgra GL_ARB_vertex_blend GL_ARB_vertex_buffer_object GL_ARB_vertex_program GL_ARB_vertex_shader GL_ARB_window_pos GL_EXT_abgr GL_EXT_bgra GL_EXT_bindable_uniform GL_EXT_blend_color GL_EXT_blend_equation_separate GL_EXT_blend_func_separate GL_EXT_blend_minmax GL_EXT_blend_subtract GL_EXT_clip_volume_hint GL_EXT_debug_label GL_EXT_debug_marker GL_EXT_depth_bounds_test GL_EXT_draw_buffers2 GL_EXT_draw_range_elements GL_EXT_fog_coord GL_EXT_framebuffer_blit GL_EXT_framebuffer_multisample GL_EXT_framebuffer_multisample_blit_scaled GL_EXT_framebuffer_object GL_EXT_framebuffer_sRGB GL_EXT_geometry_shader4 GL_EXT_gpu_program_parameters GL_EXT_gpu_shader4 GL_EXT_multi_draw_arrays GL_EXT_packed_depth_stencil GL_EXT_packed_float GL_EXT_provoking_vertex GL_EXT_rescale_normal GL_EXT_secondary_color GL_EXT_separate_specular_color GL_EXT_shadow_funcs GL_EXT_stencil_two_side GL_EXT_stencil_wrap GL_EXT_texture_array GL_EXT_texture_compression_dxt1 GL_EXT_texture_compression_s3tc GL_EXT_texture_env_add GL_EXT_texture_filter_anisotropic GL_EXT_texture_integer GL_EXT_texture_lod_bias GL_EXT_texture_mirror_clamp GL_EXT_texture_rectangle GL_EXT_texture_shared_exponent GL_EXT_texture_sRGB GL_EXT_texture_sRGB_decode GL_EXT_timer_query GL_EXT_transform_feedback GL_EXT_vertex_array_bgra GL_APPLE_aux_depth_stencil GL_APPLE_client_storage GL_APPLE_element_array GL_APPLE_fence GL_APPLE_float_pixels GL_APPLE_flush_buffer_range GL_APPLE_flush_render GL_APPLE_object_purgeable GL_APPLE_packed_pixels GL_APPLE_pixel_buffer GL_APPLE_rgb_422 GL_APPLE_row_bytes GL_APPLE_specular_vector GL_APPLE_texture_range GL_APPLE_transform_hint GL_APPLE_vertex_array_object GL_APPLE_vertex_array_range GL_APPLE_vertex_point_size GL_APPLE_vertex_program_evaluators GL_APPLE_ycbcr_422 GL_ATI_separate_stencil GL_ATI_texture_env_combine3 GL_ATI_texture_float GL_ATI_texture_mirror_once GL_IBM_rasterpos_clip GL_NV_blend_square GL_NV_conditional_render GL_NV_depth_clamp GL_NV_fog_distance GL_NV_fragment_program_option GL_NV_fragment_program2 GL_NV_light_max_exponent GL_NV_multisample_filter_hint GL_NV_point_sprite GL_NV_texgen_reflection GL_NV_texture_barrier GL_NV_vertex_program2_option GL_NV_vertex_program3 GL_SGIS_generate_mipmap GL_SGIS_texture_edge_clamp GL_SGIS_texture_lod 
Estensioni WebGL 1	ANGLE_instanced_arrays EXT_blend_minmax EXT_color_buffer_half_float EXT_frag_depth EXT_sRGB EXT_shader_texture_lod EXT_texture_filter_anisotropic MOZ_debug OES_element_index_uint OES_standard_derivatives OES_texture_float OES_texture_float_linear OES_texture_half_float OES_texture_half_float_linear OES_vertex_array_object WEBGL_color_buffer_float WEBGL_compressed_texture_s3tc WEBGL_compressed_texture_s3tc_srgb WEBGL_debug_renderer_info WEBGL_debug_shaders WEBGL_depth_texture WEBGL_draw_buffers WEBGL_lose_context MOZ_WEBGL_lose_context MOZ_WEBGL_compressed_texture_s3tc MOZ_WEBGL_depth_texture
Informazioni driver WSI WebGL 2	-
Renderer driver WebGL 2	WebGL creation failed: 
* WebGL 2 requires support for the following features: 
  transform_feedback2
* Exhausted GL driver options.
Versione driver WebGL 2	-
Estensioni driver WebGL 2	-
Estensioni WebGL 2	-
GPU #1
Attivo	Sì
ID produttore	0x10de
ID dispositivo	0x0a29
Diagnostica
AzureCanvasAccelerated	1
AzureCanvasBackend	skia
AzureContentBackend	skia
AzureFallbackCanvasBackend	none
TileHeight	512
TileWidth	512
Registro decisioni
WEBRENDER	
opt-in by default: WebRender is an opt-in feature
unavailable by runtime: Build doesn't include WebRender
Media
Back-end audio	audiounit
Numero massimo di canali	2
Layout preferito dei canali	stereo
Frequenza di campionamento preferita	44100
Dispositivi di output
Nome 	Gruppo 	Produttore 	Stato 	Preferito 	Formato 	Canali 	Frequenza 	Latenza
Altoparlanti interni	AppleHDAEngineOutput:1B,0,1,2:0	Apple Inc.	Enabled	All	default: F32LE, support: S16LE S16BE F32LE F32BE	2	default: 44100, support: 44100 - 96000	191 - 4273
Dispositivi di input
Nome 	Gruppo 	Produttore 	Stato 	Preferito 	Formato 	Canali 	Frequenza 	Latenza
Microfono interno	AppleHDAEngineInput:1B,0,1,0:1	Apple Inc.	Enabled	All	default: F32LE, support: S16LE S16BE F32LE F32BE	2	default: 44100, support: 8000 - 96000	117 - 2407
Ingresso audio	AppleHDAEngineInput:1B,0,1,1:2	Apple Inc.	Enabled	None	default: F32LE, support: S16LE S16BE F32LE F32BE	2	default: 44100, support: 8000 - 96000	117 - 2407
Preferenze importanti modificate
Nome 	Valore browser.cache.disk.capacity	358400
browser.cache.disk.filesystem_reported	1
browser.cache.disk.hashstats_reported	1
browser.cache.disk.smart_size.first_run	false
browser.cache.disk.smart_size.use_old_max	false
browser.cache.frecency_experiment	3
browser.places.smartBookmarksVersion	8
browser.sessionstore.upgradeBackup.latestBuildID	20171024165158
browser.startup.homepage_override.buildID	20171024165158
browser.startup.homepage_override.mstone	56.0.2
browser.tabs.remote.autostart.2	true
browser.urlbar.lastSuggestionsPromptDate	20171105
browser.urlbar.timesBeforeHidingSuggestionsHint	0
extensions.lastAppVersion	56.0.2
media.gmp-gmpopenh264.abi	x86_64-gcc3
media.gmp-gmpopenh264.lastUpdate	1509870152
media.gmp-gmpopenh264.version	1.6
media.gmp-manager.buildID	20171024165158
media.gmp-manager.lastCheck	1509981628
media.gmp-widevinecdm.abi	x86_64-gcc3
media.gmp-widevinecdm.lastUpdate	1509870156
media.gmp-widevinecdm.version	1.4.8.1008
media.gmp.storage.version.observed	1
network.cookie.prefsMigrated	true
network.predictor.cleaned-up	true
places.database.lastMaintenance	1509871752
places.history.expiration.transient_current_max_pages	126335
plugin.disable_full_page_plugin_for_types	application/pdf
plugins.ctprollout.cohort	test
plugins.ctprollout.cohortSample	0.206892
security.sandbox.content.tempDirSuffix	{ba0a62f4-c6df-484e-ab8a-b466cdf1da4e}
services.sync.declinedEngines	
storage.vacuum.last.index	1
storage.vacuum.last.places.sqlite	1509871746
Preferenze importanti bloccate
	Nome 	Valore
Database Places
Integrità 	
JavaScript
GC incrementale 	true
Accessibilità
Attivato 	false
Impedisci accessibilità 	0
Versioni librerie
	Versione minima prevista	Versione in uso
NSPR	4.16	4.16
NSS	3.32.1	3.32.1
NSSSMIME	3.32.1	3.32.1
NSSSSL	3.32.1	3.32.1
NSSUTIL	3.32.1	3.32.1
Caratteristiche sperimentali
Nome 	ID 	Descrizione 	Attiva 	Data termine 	Pagina iniziale 	Branch
Sandbox
Livello sandbox content process	3
Livello effettivo sandbox content process	3