To find the correct solution to your problem, we need some more non-personal information from you. Please do the following:

1. Use ONE of these methods to open the Firefox Troubleshooting Information page:
   • Click the menu button , click on help and select Troubleshooting Information.
   • Type about:support into the Firefox address bar and press the enter key.
2. At the top of the Troubleshooting Information page that comes up, you should see a button that says "Copy text to clipboard". Click it.
3. Now, go back to your forum post, right-click in the reply box and select Paste from the context menu (or else click inside the reply box and press the Ctrl+V keys) to paste all the information you copied into the forum post.

If you need further information about the Troubleshooting information page, please read the article Use the Troubleshooting Information page to help fix Firefox issues.

Thanks in advance for your help!

{

 "application": {

   "name": "Firefox",
   "version": "42.0",
   "buildID": "20151029151421",
   "userAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:42.0) Gecko/20100101 Firefox/42.0",
   "safeMode": false,
   "updateChannel": "release",
   "supportURL": "https://support.mozilla.org/1/firefox/42.0/Darwin/en-GB/",
   "numTotalWindows": 1,
   "numRemoteWindows": 0,
   "remoteAutoStart": false
 },
 "modifiedPreferences": {
   "browser.cache.disk.smart_size.first_run": false,
   "browser.cache.disk.capacity": 358400,
   "browser.cache.disk.filesystem_reported": 1,
   "browser.cache.frecency_experiment": 2,
   "browser.download.importedFromSqlite": true,
   "browser.places.smartBookmarksVersion": 7,
   "browser.privatebrowsing.autostart": true,
   "browser.sessionstore.upgradeBackup.latestBuildID": "20151029151421",
   "browser.startup.homepage_override.mstone": "42.0",
   "browser.startup.homepage": "google.com",
   "browser.startup.homepage_override.buildID": "20151029151421",
   "dom.apps.reset-permissions": true,
   "dom.mozApps.used": true,
   "extensions.lastAppVersion": "42.0",
   "gfx.blacklist.direct2d": 3,
   "media.gmp-manager.buildID": "20151029151421",
   "media.gmp-gmpopenh264.lastUpdate": 1450044268,
   "media.gmp-gmpopenh264.version": "1.5.1",
   "media.gmp-gmpopenh264.abi": "x86_64-gcc3-u-i386-x86_64",
   "media.gmp-gmpopenh264.enabled": false,
   "media.gmp-manager.lastCheck": 1450130912,
   "network.cookie.prefsMigrated": true,
   "network.predictor.cleaned-up": true,
   "places.history.expiration.transient_current_max_pages": 104858,
   "places.database.lastMaintenance": 1450044585,
   "plugin.importedState": true,
   "plugin.disable_full_page_plugin_for_types": "application/pdf",
   "privacy.sanitize.migrateFx3Prefs": true,
   "storage.vacuum.last.places.sqlite": 1450044585,
   "storage.vacuum.last.index": 0
 },
 "lockedPreferences": {},
 "graphics": {
   "numTotalWindows": 1,
   "numAcceleratedWindows": 1,
   "windowLayerManagerType": "OpenGL",
   "windowLayerManagerRemote": true,
   "supportsHardwareH264": false,
   "adapterDescription": "",
   "adapterVendorID": "0x8086",
   "adapterDeviceID": "0x0a2e",
   "adapterRAM": "",
   "adapterDrivers": "",
   "driverVersion": "",
   "driverDate": "",
   "webglRenderer": "Intel Inc. -- Intel Iris OpenGL Engine",
   "info": {
     "AzureCanvasBackend": "skia",
     "AzureSkiaAccelerated": 0,
     "AzureFallbackCanvasBackend": "none",
     "AzureContentBackend": "quartz"
   }
 },
 "javaScript": {
   "incrementalGCEnabled": true
 },
 "accessibility": {
   "isActive": false,
   "forceDisabled": 0
 },
 "libraryVersions": {
   "NSPR": {
     "minVersion": "4.10.10",
     "version": "4.10.10"
   },
   "NSS": {
     "minVersion": "3.19.4 Basic ECC",
     "version": "3.19.4 Basic ECC"
   },
   "NSSUTIL": {
     "minVersion": "3.19.4",
     "version": "3.19.4"
   },
   "NSSSSL": {
     "minVersion": "3.19.4 Basic ECC",
     "version": "3.19.4 Basic ECC"
   },
   "NSSSMIME": {
     "minVersion": "3.19.4 Basic ECC",
     "version": "3.19.4 Basic ECC"
   }
 },
 "userJS": {
   "exists": false
 },
 "crashes": {
   "submitted": [],
   "pending": 0
 },
 "extensions": [
   {
     "name": "Adobe Acrobat - Create PDF",
     "version": "2.0",
     "isActive": false,
     "id": "web2pdfextension@web2pdf.adobedotcom"
   }
 ],
 "experiments": []

}

all this? thanks anyway...

If you scroll down any page on Facebook, additional content loads into the tab. Depending on the content of the news feed/timeline, memory use keeps growing. As far as I know, Firefox cannot release any of the memory used by this page as long as the tab is open and even if you navigate away, Firefox keeps a pre-rendered version of the page in memory as part of its "fast back-forward cache" to speed up display if you use the Back button. So... Facebook can definitely use a lot of memory on Firefox.

That said, I'm not sure 1GB of memory use would cause Firefox to become slow; I guess it depends on your system.

But more generally, does anything in this article help: Quick fixes if your Firefox slows down.