Do you have that problem with the TabMix Plus extension disabled? A test to see if TMP is causing that fault.

If not a problem with it disabled, see the TMP forum about that issue. http://tmp.garyr.net/forum/

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of your add-ons is causing your problem (switch to the DEFAULT theme: Tools > Add-ons > Themes). See Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems and Troubleshoot issues with plugins like Flash or Java to fix common Firefox problems

If it does work in Safe-mode then disable all your extensions and then try to find which is causing it by enabling one at a time until the problem reappears. You can use "Disable all add-ons" on the Safe mode start window to disable all extensions. You have to close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

Thanks for the suggestions. I posted a follow-up to my original post last night that I came to suspect TMP myself and found that an update that hadn't yet been automatically installed fixed the problem. Don't know where the follow-up post landed.

At any rate, the problem is solved. Again, thanks for the help!