Clear the Cache and remove the Cookies for websites that cause problems via the "3-bar" Firefox menu button (Settings). Start Firefox in Troubleshoot Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration or userChrome. css is causing the problem.

Is this a problem with web mail in Firefox or possibly with Thunderbird?

If the question is about Thunderbird then we can move the question thread to Thunderbird Support.

Can you attach a screenshot?

• https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem
• use a compressed image type like PNG or JPG to save the screenshot