hello, this is a google issue. on their support site they say that all current versions of firefox are supported but apparently something has gone wrong in there browser sniffing which will have to be corrected on their server side...

http://productforums.google.com/d/topic/google-plus-discuss/QvaQaWgYuQE/discussion

Thank you phillipp, but that didn't help.

Hello Firefoxhelper123, confirm philipp's above reply, see also :

https://support.mozilla.org/en-US/questions/967566

thank you

This didn't help, and there is no use for the user agent.

By the way ideao and everyone else, none of hose things helped, it figured itself out. I assumed something wasn't right at the time.

My apologies to the question owner and others. This response does not solve the immediate problem and won't help but having wasted several hours trying to figure out what was going on and suspecting a ploy to direct me away from Firefox and use Google Chrome instead, I am not feeling particularly charitable.

I have therefore uninstalled Google Chrome and other things Google. In Google's uninstalling questionnaires I made it clear that it was because their search engine was no longer not compatible with Firefox.

I know I am cutting my nose of to spite my face but that serves me right for becoming too dependent on Google's search engine.

Perhaps if more people uninstall Google Chrome and say why, Google may be prompted to do something about the problem more quickly!

Moderator - locking this thread because the Owner of the thread solved the issue himself - unknown cause. This thread has no further value for other users who may have the same or a similar problem with G+.