hello callender, please enter about:config into the location bar, search for the preference named network.http.accept-encoding & right click and reset that to its default value. this should make gmail load normally again...

Brilliant - that worked - many thanks. Tony

doesn't work for me

It still works, but can someone explain what the cause of this problem is?

I am now on a new release of FireFox (17.0.1) and I still have to perform this task every time that open the browser.

Thanks Tony

hello tony, you should only have to change the setting once! if the preference changes back automatically each time you restart firefox then something external is interfering here (maybe some sort of tune-up software). in order to troubleshoot that, please see How to fix preferences that won't save - especially the part about the user.js file in your profile folder.

edit: please also Update Firefox to the latest release to stay safe on the web!