Clearing cache helps in many cases, but with Google, you might also need to remove the site's cookies.

Save any pending work first, then, while viewing a page on the site, try either:

• right-click and choose View Page Info > Security > "View Cookies"
• Alt+t (open the classic Tools menu) > Page Info > Security > "View Cookies"

In the dialog that opens, you can remove the site's cookies individually.

Then try reloading the page and logging back in to Google+.

Any improvement?

Thanks for the suggestions. I'm afraid it made no difference though.

Could you also try Firefox's Safe Mode? That's a standard diagnostic tool to bypass interference by extensions (and some custom settings). More info: Use Troubleshoot Mode in Firefox.

You can restart Firefox in Safe Mode using

Help > Restart with Add-ons Disabled

In the dialog, click "Start in Safe Mode" (not Reset)

Any difference?

If you use extensions (Firefox/Tools > Add-ons > Extensions) that can block content (e.g. Adblock Plus, NoScript, Flash Block, Ghostery) then make sure that such extensions aren't blocking content.

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Thanks. That solves the problem. I think it must be Adblock Plus.

Thanks. That was the solution.