hello, could you go to the firefox menu ≡ > help ? > troubleshooting information, copy the contents of that page and paste them here into a reply on the forum? this might give us a clue what is going on...

Thank you. I have copied the raw data, which is...

{

 "application": {
   "name": "Firefox",
   "version": "34.0",
   "userAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:34.0) Gecko/20100101 Firefox/34.0",
   "supportURL": "https://support.mozilla.org/1/firefox/34.0/Darwin/en-US/",
   "numTotalWindows": 1,
   "numRemoteWindows": 0
 },
 "crashes": {
   "submitted": [],
   "pending": 0
 },
 "modifiedPreferences": {
   "browser.cache.disk.capacity": 358400,
   "browser.cache.disk.smart_size.first_run": false,
   "browser.cache.disk.smart_size_cached_value": 358400,
   "browser.cache.disk.smart_size.use_old_max": false,
   "browser.cache.frecency_experiment": 4,
   "browser.places.smartBookmarksVersion": 7,
   "browser.sessionstore.upgradeBackup.latestBuildID": "20141125180439",
   "browser.startup.homepage_override.mstone": "34.0",
   "browser.startup.homepage_override.buildID": "20141125180439",
   "browser.tabs.warnOnClose": false,
   "browser.tabs.loadInBackground": false,
   "browser.tabs.warnOnOpen": false,
   "dom.w3c_touch_events.expose": false,
   "dom.mozApps.used": true,
   "extensions.lastAppVersion": "34.0",
   "font.internaluseonly.changed": false,
   "media.gmp-gmpopenh264.lastUpdate": 1413442906,
   "media.gmp-gmpopenh264.version": "1.1",
   "media.gmp-manager.lastCheck": 1417716296,
   "network.cookie.prefsMigrated": true,
   "places.history.expiration.transient_current_max_pages": 104858,
   "places.database.lastMaintenance": 1417779606,
   "plugin.state.quicktime plugin": 2,
   "plugin.disable_full_page_plugin_for_types": "application/pdf",
   "plugin.state.java": 2,
   "plugin.importedState": true,
   "print.print_plex_name": "",
   "print.print_margin_top": "0.5",
   "print.print_paper_width": "  8.50",
   "print.print_resolution_name": "",
   "print.print_orientation": 0,
   "print.print_to_file": false,
   "print.print_paper_height": " 11.00",
   "print.print_scaling": "  1.00",
   "print.print_unwriteable_margin_right": 13,
   "print.print_command": "",
   "print.print_duplex": 0,
   "print.print_reversed": false,
   "print.print_unwriteable_margin_left": 13,
   "print.print_unwriteable_margin_bottom": 20,
   "print.print_paper_size_unit": 0,
   "print.print_margin_right": "0.5",
   "print.print_oddpages": true,
   "print.print_bgcolor": true,
   "print.print_bgimages": true,
   "print.print_downloadfonts": false,
   "print.print_evenpages": true,
   "print.print_margin_left": "0.5",
   "print.print_colorspace": "",
   "print.print_shrink_to_fit": true,
   "print.print_unwriteable_margin_top": 12,
   "print.print_paper_name": "",
   "print.print_margin_bottom": "0.5",
   "print.print_in_color": true,
   "print.print_paper_size_type": 1,
   "print.print_page_delay": 50,
   "print.print_paper_data": 0,
   "print.print_resolution": 0,
   "privacy.sanitize.timeSpan": 0,
   "privacy.sanitize.migrateFx3Prefs": true,
   "privacy.cpd.offlineApps": true,
   "security.warn_viewing_mixed": false,
   "security.disable_button.openDeviceManager": false,
   "storage.vacuum.last.places.sqlite": 1416046289,
   "storage.vacuum.last.index": 1
 },
 "lockedPreferences": {},
 "graphics": {
   "numTotalWindows": 1,
   "numAcceleratedWindows": 1,
   "windowLayerManagerType": "OpenGL",
   "windowLayerManagerRemote": true,
   "adapterDescription": "",
   "adapterVendorID": "0x8086",
   "adapterDeviceID": "0x 166",
   "adapterRAM": "",
   "adapterDrivers": "",
   "driverVersion": "",
   "driverDate": "",
   "webglRenderer": "Intel Inc. -- Intel HD Graphics 4000 OpenGL Engine",
   "info": {
     "AzureCanvasBackend": "quartz",
     "AzureSkiaAccelerated": 0,
     "AzureFallbackCanvasBackend": "none",
     "AzureContentBackend": "quartz"
   }
 },
 "javaScript": {
   "incrementalGCEnabled": true
 },
 "accessibility": {
   "isActive": false,
   "forceDisabled": 0
 },
 "libraryVersions": {
   "NSPR": {
     "minVersion": "4.10.7",
     "version": "4.10.7"
   },
   "NSS": {
     "minVersion": "3.17.2 Basic ECC",
     "version": "3.17.2 Basic ECC"
   },
   "NSSUTIL": {
     "minVersion": "3.17.2",
     "version": "3.17.2"
   },
   "NSSSSL": {
     "minVersion": "3.17.2 Basic ECC",
     "version": "3.17.2 Basic ECC"
   },
   "NSSSMIME": {
     "minVersion": "3.17.2 Basic ECC",
     "version": "3.17.2 Basic ECC"
   }
 },
 "userJS": {
   "exists": false
 },
 "extensions": [
   {
     "name": "Adblock Edge",
     "version": "2.1.6",
     "isActive": true,
     "id": "{fe272bd1-5f76-4ea4-8501-a05d35d823fc}"
   },
   {
     "name": "Adblock Plus",
     "version": "2.6.6",
     "isActive": true,
     "id": "{d10d0bf8-f5b5-c8b4-a8b2-2b9879e08c5d}"
   },
   {
     "name": "Flashblock",
     "version": "1.5.17",
     "isActive": true,
     "id": "{3d7eb24f-2740-49df-8937-200b1cc08f8a}"
   }
 ],
 "experiments": []

}

I do use Adblocker plus, as go a bit mental after onslaught of pop-up and other junk.

i think the flashblock addon is known to cause issues on the new version, please try disabling it. as an alternative you could use an addon like https://addons.mozilla.org/firefox/addon/click-to-play-per-element/

Thanks Philip, Is the Flashblock add on part of Adblocker, or bit of Firefox?

I can try opening Adblocker on Facebook and Youtube, otherwise will have to revert to that clunking horror, Safari.

There is an 'unblocker' available from Firefox/Mozilla, which allows you to watch 'blocked videos on Youtube and Facebook', but it is currently blocked - by Firefox!

I think I'll have to switch to Safari, as I'm not up to reconfiguring the entire flipping thing. Thanks for your help though, and if you have a direct line to Firefox, a message along the lines of 'If it ain't broken...'.