Mac/Firefox weirdness

[#1]

Long shot...
Anyone got any ideas why the Mozilla site suddenly seems to think I'm running FF6 (and thus wont let me install any more plugins)?
Actually running FF25.
(Noticed that Netalyzr also reports as running FF6.)
What goes on?!
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edit -
It gets weirder.
The troubleshooter for FF thinks I'm running Windows7. (Actually Mac OS 10.6.8.)
Posted on Mozilla forums - wait for response....

[#2]

Weird, I'm running FF 25 and it reports OK.
Have you tried a reinstall?

[#3]

Check the user agent string in about:config
It may have been set by an add-on to something else other than default.

[#4]

Not sure what I'm looking for in config, but its more than FF.
Netalyzr and Mozilla think I'm running Windows7, I'm actually on an iMac, so its a bit more basic, and I'm getting worried.  
Most recent likely offender is an MP3 converter add-on for Audacity, but I'm out of my depth here.
Have posted to Apple Communities.
Any ideas if there's something I can look at via the Terminal that might give me a pointer?

[#5]

Try entering this Javascript into the location bar and hit return or go if that's still there these days, it will print out what your user agent is reporting to sites you're visiting.

 javascript:document.write(navigator.userAgent)

It should come up like my one below.
Mozilla/5.0 (Macintosh; Intel Mac OS X 10.6; rv:24.0) Gecko/20100101 Firefox/24.0

[#6]

Cool.
You were right, FF config is showing -
general.useragent.override - Mozilla/5.0 (Windows NT 6.1; rv:6.0) Gecko/20100101 Firefox/6.0
Once I have the correct user agent, how do I edit the config file?

[#7]

Well it might help to find out what set it in the first place as it may come back later. Have you got an add-on in Firefox called "Useragent switcher"?
To remove it you'll have to open up prefs.js or user.js in TextEdit or similar editor (located in ~/Library/Application Support/Firefox/Profiles/"nums & letters".default). Search for "general.useragent.override" and delete that whole line (check user.js first) all while Firefox is closed/has been quit.

[#8]

No worries.
Got reply link from Mozilla on easy resetting incorrect useragent in config*. All done.  
Re what did it in the first place... no idea. No, don't use a switcher. I'll keep an eye on it in future though!
Many thanks. Considerable relief, thought something worse had happen for a minute there!
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* https://support.mozilla.org/en-US/kb/websites-say-firefox-outdated-or-incompatible