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

You can try to disable hardware acceleration in Firefox.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

You need to close and restart Firefox after toggling this setting.

• https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration

toomanylogins said

After update to 38.0.1 I have the menu toolbar enabled and I have a problem with the firefox window changing size when viewing pages. The whole window jitters then I lose the menu bar at the top of the tab bar. at this point I cannot click or enter data into a web page. If I select the border of the window and resize or move the window returns to its original size with the menu bar and tabs correctly displayed. I also have Firefox developer v 40.a02 this was OK last week I was updated on 29/05 and now also has the same problem.

Disable hardware acceelration seems to work.