You also can use the classic menu bar. Tap the Alt key to display it if you don't normally use it. Then

Tools > Options

In the left column, click Content.

If you are on a site where you want to allow or block popup windows, you also can access that using the Permissions panel of the Page Info dialog. You can call that up using any of these:

• right-click a blank area of the page and choose View Page Info > Permissions
• (menu bar) Tools menu > Page Info > Permissions
• click the padlock or globe icon to the left of the site address > click the > and then More Information > Permissions

If the popup blocker seems to have stopped working on a site, it's possible that the site is using in-page overlays instead of separate windows. The popup blocker is only concerned with new windows.

Classic Theme Restorer (Customize Australis) {web link} Restore squared tabs, appmenu, add-ons bar, small button view and more on Australis UI (Windows/MacOSX/Linux + Fx 29+).

This will restore many of the things the last upgrades took away.

http://forums.mozillazine.org/viewtopic.php?f=48&t=2827985 Classic Theme Restorer - Discussion Thread (v2)

You can alternatively open the about:preferences page in a new tab and bookmark this page.

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

• Switch to the DEFAULT theme: Firefox menu button/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