Try to refresh firefox going to menu>help>troubleshooting infotmation>refresh firefoxTheDrCK said

Over the past two days or so, Firefox has been loading pages incorrectly (e.g. loading mobile versions of sites despite using Firefox on my desktop PC). I'm using Firefox 38.0.5 in Windows 7, but the browser's user agent occasionally reads as Chrome 39 on Linux or Chrome 40 on iOS, with the general.useragent.override string listed variously as things like 'Mozilla/5.0 (X11; Linux i686) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.71 Safari/537.36', 'Mozilla/5.0 (Macintosh; Intel Mac OS X 10_6_8) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/40.0.2214.93 Safari/537.36', and 'Mozilla/5.0 (iPhone; CPU iPhone OS 8_3 like Mac OS X) AppleWebKit/600.1.4 (KHTML, like Gecko) CriOS/43.0.2357.56 Mobile/12F70 Safari/600.1.4 (000602)'. Whenever I change this string manually to 'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:29.0) Gecko/20100101 Firefox/38.0.5', it changes back. I'm yet to figure out what causes this switch; the timing is triggers are inconsistent. I've tried adding the User Agent Overrrider addon and setting the extension's user agent to 'Mozilla/5.0 (Windows NT 6.1; WOW64; rv:29.0) Gecko/20100101 Firefox/38.0.5'. For whatever reason, the browser reverts back to Chrome after a period of time, despite UAO being explicitly set to Firefox 38. I've tried deleting user.js and prefs.js, but neither of these approaches worked. I thought it possible that an extension might be causing trouble, but I've not installed any new extensions for a long while, and all are pretty innocuous: Classic Theme Restorer, Dictionary, Google Analytics Opt-out, Greasemonkey, RES, uBlock, Youtube Centre, Zotero. Of these, only Classic Theme Restorer has updated itself in the past two days,yet the problem persists when this extension is removed. Nothing dubious shows up in Malwarebytes or Aunruns. Any idea how I can find out what's doing this?

The general.useragent.override pref isn't there by default and the fact that you have it already means that a Firefox extension or other software is creating this pref.

Do you have a user.js file in the profile folder?

If that is the case then can you you post its content?

Note that the user.js file shouldn't be there as well just like the general.useragent.override pref.

See:

• http://kb.mozillazine.org/Preferences_not_saved