Try clearing your browser cache.

Tools > Clear Recent History... - hit Details and make sure only Cache is selected, then select Everything and hit the Clear Now button.

Sorry old chap that was the very first thing I did. I've searched around for reasons and I reckon it's something to do with a security issue that Mozilla has tried to fix.

Consider this message which appeared in the error console, amongst others:

Security Error: Content at https://mail.google.com/a/mydomain.com/x/cjjufmvo71sn/ may not load data from https://sites.google.com/a/mydomain.com/home/.

Can anyone suggest a solution (about:config switch?)

Update: I should have pointed out that I missed updates 3.6.9 and 3.6.10 and so my last working version was 3.6.8. I have done a cascade of reversions back through 10 and 9 and these two also fail. It's therefore clear that 3.6.9 is the faulty one.

Update: Somehow my FF version has auto-upgraded to 3.6.12 even though I had the browser settings set to not upgrade. Now everything has stopped working. I can't find version 3.6.8 in the list of available downloads. Where has it gone please?

...and trying the "Search History" option now makes FF hang, so I can't even find the 3.6.8 download page that way.

That site uses X-Frame-Options: SAMEORIGIN and that makes it impossible to open such a page in an IFrame. Firefox started supporting X-Frame-Options with the 3.6.9 version.

• http://michael-coates.blogspot.com/2010/08/x-frame-option-support-in-firefox.html
• http://developer.mozilla.org/en/The_X-FRAME-OPTIONS_response_header

Cor-el:

Thank you very much for confirming my suspicions. I've just had to find and download a dodgy copy of FF3.6.8 to get my home pages working again.

Are you sure there isn't an about:config switch to turn this "feature" off?

I have posted this issue to the Google forum because clearly mail.google.com and sites.google.com are within the same domain and should surely be accepted.