Try the Firefox Safe Mode to see how it works there. The Safe Mode is a troubleshooting mode, which disables most add-ons.

(If you're not using it, switch to the Default theme.)

• You can open the Firefox 4.0+ Safe Mode by holding the Shift key when you use the Firefox desktop or Start menu shortcut.
• Or use the Help menu item and click on the Restart with Add-ons Disabled... menu item while Firefox is running.

Don't select anything right now, just use "Continue in Safe Mode." To exit the Firefox Safe Mode, just close Firefox and wait a few seconds before using the Firefox shortcut (without the Shift key) to open it again. If it is good in the Firefox Safe Mode, your problem is probably caused by an extension, and you need to figure out which one.

Please follow the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article for that.

When you figure out what's causing your issues, please let us know. It might help other users who have the same problem.

I also had the same problem with Firefox 16.0.1 and restarted the browser in safe mode. In safe mode the problem was gone and I was able to change the display density in Gmail. It appears as though the offending plugin was Adblock Plus as when I started the browser with plugins enabled and disabled Adblock Plus the problem was fixed as well. The only two plugins I have installed are Adblock Plus and VideoDownload Helper.

Thanks so much for your help. This worked.

G