Redirect Loop
Firefox has detected that the server is redirecting the request for
this address in a way that will never complete.
The browser has stopped trying to retrieve the requested item. The
site is redirecting the request in a way that will never complete.
* NOTE: If accepting the site's cookies does not resolve the
problem, it is likely a server configuration issue and not your
computer.
Here is a screenshot of the same:

This was driving me nuts. I tried some obvious things like restarting firefox, rebooting my machine. Needless to say none of them fixed the problem.
Then I started looking on internet for any fixes and all of them talked about cookies. One of the recommended fix that I saw over and over was to clear cookies. The steps are as follows:
Tools > Options > Privacy > Clear Now > Make sure that cookies option is checked before you clear data.
This fix did work but it was temoporary relief. The error was back once I close the GMail tab. Upon further digging through the forums I found somewhere that it this error was caused by a Firefox add-on but they did nto mention whichone. My initial suspects were all add-on's I have that concern with GMail and cookies like 'Better GMail 2', 'GMail Manager', 'FireCookie' etc. I started going through the reviews for these add-on's to find any mention of this error.
Bingo, I found that 0.5.7 version of 'GMail Manager' was responsible for this problem. The author put out a new version 0.5.7.1 which can he found here.
Hope this is helpful.
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