If I'm not at the computer, or doing something in a different window, then go to another window, as soon as the mouse hits the page, I get the spinning ball, and must wait 20 secs to a minute, until it will refresh the page, or I can then do anything. This is very aggravating & getting old, as it's occurred in 17, 18, 19, and now in the Beta 20. All my plug ins are up to date, and I keep hoping each new update will end this issue.
Could you try disabling graphics hardware acceleration? Since this feature was added to Firefox, it has gradually improved, but there still are a few glitches.
You might need to restart Firefox in order for this to take effect, so save all work first (e.g., mail you are composing, online documents you're editing, etc.). Then perform these steps:
Additionally, please check for updates for your graphics driver by following the steps mentioned in the following Knowledge base articles:
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On Mac disable hardware acceleration in Firefox here:
Try to do a clean install of Firefox and download the full version and trash the currently installed version.
Download a new copy of the Firefox program and save the disk image (dmg) file to the desktop
Your profile data is stored elsewhere in the Firefox Profile Folder, so you won't lose your bookmarks and other personal data if you uninstall and (re)install Firefox.
Try to create a new profile as a test to check if your current profile is causing the problems.
See "Creating a profile":
If the new profile works then you can transfer some files from an existing profile to the new profile, but be careful not to copy corrupted files.
Did that, new install, still stalling on every new window/tab, will wait for a new version, and once again, see if it works any better...