Did you post from your Mac? Your "user agent" text is:

Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.3) Gecko/20100101 Ubuntu/8.04 Firefox/4.0

Assuming that's incorrect, check out this article for how to fix it: How to reset the default user agent on Firefox.

Any luck?

Oh wait, you have an extension named "randomUserAgent". If that serves a useful function on other sites, maybe you need to make an exception for Google.

I disables the randomUserAgent extension in Firefox and still Gmail only opens in basic HTML. Any other thoughts? I've successfully used the RandomUserAgent extension for a month or two, it auto updated a few weeks ago. But this afternoon suddenly Gmail started giving me the error message: "Some important features may not work in this version of your browser, so you have been redirected to the Basic HTML version. Upgrade to a modern browser, such as Google Chrome." When I follow the Google "Supported browser" link, the link to "Firefox download" will only partially open . . .

Try the support article linked above to manually repair your user agent string.

I did the manual repair as described and now my AOL mail (as well as Gmail) only open in Basic HTML. AOL has a similar message about the Firfox browser being unsupported . . . Do I need to reinstall the latest Firefox??

BTW I used my iMac

Hi RufusJones, if you check the following dialog on the problem system, what version does it show?

Help > About Firefox

If you check the Troubleshooting screen, what is listed as "User Agent" (third line down in the first table)?

Help > Troubleshooting Information

Firefox 22.0

Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.3) Gecko/2008102920 Ubuntu/8.04 Firefox/3.0.1

Hi RufusJones, your Firefox seems to be getting older!

Did you restart Firefox after disabling the random user agent extension? If not, please do that now. There should be a link above the extension for that purpose.

Then, unfortunately, you would need to reset those user agent preferences one more time and check Troubleshooting Information again to make sure you are back to Firefox 22 on a Mac.

Hope it works this time.

Yes, I both restarted Firefox and rebooted the computer.

Will reset the agent preferences but the trouble shooting page says that I am on Firefox version 22.0.

If all else fails, should I "Reset Firefox" on the trouble shooting page?

There is a broken user agent override (rv:1.9.0.3=Firefox 3.6.3):

• general.useragent.override: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.0.3) Gecko/20100101 Ubuntu/8.04 Firefox/4.0

The support URL suggests a Mac:

• Support URL: http://support.mozilla.org/1/firefox/22.0/Darwin/en-US/

It is usually a bad idea to have a user agent of another platform because that can cause some servers to send wrong code and Firefox 4.0 is also really old.

Hi RufusJones, I don't think you will need to use Firefox Reset, but that does remain an option if we can find any other way to resolve it.

You are loosing me ... I disable the "RandomUserAgent" extension in Firefox.

Are you saying "reset those user agent preferences one more time and check Troubleshooting Information again to make sure you are back to Firefox 22 on a Mac. " or do something else? Like I said, Troubleshooting says that I'm 22.

Everything back to normal, thank you very much jscher2000!