Your System Details list shows that you likely have a corrupted user agent that identifies you as Firefox/3.5.7:

• Mozilla/5.0 (Windows; U; Windows NT 6.0; en-US; rv:1.9.1.7) Gecko/20091221 Firefox/3.5.7 (.NET CLR 3.5.30729) FBSMTWB

You can reset all user set (bold) general.useragent prefs on the about:config page via the right-click context menu to the default value.

See:

• https://support.mozilla.org/kb/Finding+your+Firefox+version
• https://support.mozilla.org/kb/websites-say-firefox-outdated-or-incompatible

Hi, cor-el. If I may ask, could this be fixed by downloading the full installer?

No, you need to clear this up in the Firefox profile folder or on the about:config page in Firefox.

All you need to do is to open the about:config page via the location bar and accept the warning and click "I'll be careful" to continue.

In the search field you can type agent and reset at least the general.useragent.override pref via the right-click context menu to the default value and possible other general.useragent that show user set (bold).

See also:

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

You can use this button to go to the currently used Firefox profile folder:

• Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)