Whom are you calling for this help and where did you download Firefox from? So if there nothing important in Firefox then you should uninstall ND delete the mozilla folder then try again. Did you verify Firefox want set to offline mode as well.

I just had this problem as well - all other browsers were working, but not Firefox - it kept on acting as if there was no internet connection. I am on Linux (Debian) and from one moment to the next, Firefox was unable to load any websites (even ones hosted locally on the machine btw.).

None of the troubleshootings available online helped: - checking/disabling the proxy settings - no effect - disabling ipv6 vor dns (about:config - network.dns.disableIPv6 /disablePrefetch) - no effect - clearing cache, cookies, settings... - no effect.

Here is what fixed it: In the system settings every application has settings what resources it can access on the machine. For some reason, Firefox had lost its rights to access the network. (Seems simple - but took me 2h to find.)