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")

This is a hidden add on! Here’s the easy, get rid of it for good solution: It’s done within Firefox. Enter [about:config] in the url address box. Click “I’ll be careful. I promise”. In the <filter> field, type “extensions.netassistant” (no quotes). Right click on the field: “extensions.netassistant.tabsearch.url”, click on [modify] , either delete its string or enter your own. I’ve entered “https://encrypted.google.com” (this is a beta encrypted search by Google) just as a personal satisfaction against Yahoo. Do the same for:”extensions.netassistant.url” field. NOTE: If you haven’t modified your Firefox configuration before and you don’t feel comfortable in what you are about to do, BACKUP! The backup procedure can be found at: http://support.mozilla.com/en-US/kb/backing%20up%20your%20information . Or just do a Google search for “backup Firefox pref.js”.

When I tried your method the - "extensions.netassistant" didn't show up in the list? Why not? Did I do something wrong? I copied the text from your post so I could be sure I didn't misspell it (as I did here in this post and the system is telling me that the spell checker things it is spelled wrong) but when I got there it did not show up? Is it in fact still there? Please help as I hate Yahoo as you do. They shouldn't be able to put something on your computer that you don't want and can't get rid of easily!