You posted this earlier today in an older thread on the Thunderbird support forum :

https://support.mozilla.org/en-US/questions/1215339

Did you do what was advised there  ?

The header of your question here (the support forum for Firefox) reads :

"I'm offline. How do I get back online? "

Press the Alt ley to bring up Firefox menu - click 'File' => uncheck 'Work Offline'.

That should do the trick ......

What about the rest of the computers programs? Are they off-line also?

Some problems occur when your Internet security program was set to trust the previous version of Firefox, but no longer recognizes your updated version as trusted. Now how to fix the problem: To allow Firefox to connect to the Internet again;

• Make sure your Internet security software is up-to-date (i.e. you are running the latest version).
• Remove Firefox from your program's list of trusted or recognized programs. For detailed instructions, see

Configure firewalls so that Firefox can access the Internet. {web link}