Clear the cache and cookies.

To clear cache and cookies do the following:

1. Go to Firefox > History > Clear recent history or (if no Firefox button is shown) go to Tools > Clear recent history.
2. Under "Time range to clear", select "Everything".
3. Now, click the arrow next to Details to toggle the Details list active.
4. From the details list, check Cache and Cookies and uncheck everything else.
5. Now click the Clear now button.

Further information can be found in the Clear your cache, history and other personal information in Firefox article.

I did follow directions and did a restart of Firefox . Same problem remains. It's not a big problem. The system seems to work OK except for this inability to change size of mail display.

Can you try this in safe mode, it may be a addon affecting this.

• Diagnose Firefox issues using Troubleshoot Mode

Yes, you were correct. It was an add on. Thank you very much.