Please first try a new profile to make sure that its not an issue with anything that was added to the default Firefox.

• Profile Manager - Create, remove or switch Firefox profiles

Second please try this: Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems

• Upgrade your graphics drivers to use hardware acceleration and WebGL

To make sure it is not mixed content:

• Websites look wrong or appear differently than they should

Please post back if none of them work, I am happy to investigate further.

You can try these steps in case of issues with web pages:

You can reload web page(s) and bypass the cache to refresh possibly outdated or corrupted files.

• Hold down the Shift key and left-click the Reload button
• Press "Ctrl + F5" or press "Ctrl + Shift + R" (Windows,Linux)
• Press "Command + Shift + R" (Mac)

Clear the cache and remove cookies only from websites that cause problems.

"Clear the Cache":

• Firefox/Tools > Options > Advanced > Network > Cached Web Content: "Clear Now"

"Remove Cookies" from sites causing problems:

• Firefox/Tools > Options > Privacy > "Use custom settings for history" > Cookies: "Show Cookies"

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

cjm_dbm said

On 2 different computers we are having this problem, just in the past few days. It actually started on one computer a few days before it started on the other. Today we checked and the problem does not happen on INTERNET EXPLORER (which we NEVER USE, we always use FIREFOX). We have been using FIREFOX for years and have not had this issue previously. Here is the problem - When we open an email, the rest of the YAHOO email page goes away, all the control buttons right above the emails (delete, move, etc) and all the YAHOO titles above the email (search, news, games, etc). Also the bottom half of the screen (within the message) goes blank/gray. When we return to the inbox, that is missing all the page display top and bottom as well. The only thing that corrects it is if we select "YAHOO MAIL" again from our browser toolbar bookmarks, and then it only stays correct for a moment or so. We deleted all of our history and cache, several different times and each time we do that it corrects for a few moments and then goes right back. We cannot get help on YAHOO because when we followed all the instruction there it came back that it was a browser problem, that is when we checked INTERNET EXPLORER and found that it was fine on that. PLEASE HELP! This is a very aggravating problem and we do not want to switch to another browser.

Please notice in first couple lines of description of problem that this problem is happening on 2 totally different computers, with 2 different profiles. Also Cache and Cookies have been cleared several times.

Resetting/Refreshing/Reloading the page using the shift key only worked for a moment, just as it had when we had refreshed the page all the other times. It is okay for a moment, but then goes back to cutting the bottoms off. And again, ALL cache and cookies have been cleared several times, as well as closing and re-opening Mozilla (and we have Mozilla set to clear all history, cache and cookies each time we close it) and completely shutting down the computer and restarting it. The inbox comes back looking normal until going into one of the emails and then everything is cut off on the bottom again even in the inbox. All this is exactly the same on both computers.