Can you install Sherlock plugins or does that fail as well?

• http://mycroft.mozdev.org/google-search-plugins.html

Clear the cache and the cookies from sites that cause problems.

"Clear the Cache":

• Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"

"Remove Cookies" from sites causing problems:

• Tools > Options > Privacy > Cookies: "Show Cookies"

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance/Themes).

• Don't make any changes on the Safe mode start window.
• https://support.mozilla.org/kb/Safe+Mode

Thank you very much Cor-el! I started FF in safe mode to rip the add-ons out of the mix, and the new search engines I wanted to add installed perfectly. I like my mix of add-ons so I'm keeping them, but will use this workaround whenever I add new search engines going forward. Excellent advice, thanks again C!

You're welcome.

If it does work in Safe-mode then disable all extensions and then try to find which is causing it by enabling one at a time until the problem reappears.

• Use "Disable all add-ons" on the Safe mode start window to disable all extensions.
• Close and restart Firefox after each change via "File > Exit" (Mac: "Firefox > Quit"; Linux: "File > Quit")

• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes