This issue can be caused by an extension that isn't working properly.

Start Firefox in Diagnose Firefox issues using Troubleshoot Mode to check if one of the extensions 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.com/kb/Safe+Mode

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

Problem not solved. I recently downloaded a file umpacking utility called "jzip". It added a new homepage that I can't remove. Home is now http://search.jzip.com/

I have a feeling this is the cause of Firefox not operating properly. I changed home page to msnbc.com but after reload it comes up as search.jzip.com/

Uninstall jZip via the Control Panel.
If you want to keep the program then do a custom install and make sure that you do not install browser components.

• /questions/836508
• /questions/805889

Reset all jZip related prefs on the about:config page.

You can open the about:config page via the location bar and do a search for jZip via the Filter at the top of the about:config page.
You can reset all jZip related prefs via the right-click context menu to their default values.

When I type about:config into address, it searches the web for that topic instead of activity the utility.

If I start in Safe Mode, I can go into about:config and reset to default home page. Everything works OK until I reload Firefox. Reloading in Safe Mode, everything works but jzip is still my home page. In Normal Mode, the browser doesn't function properly again. My bookmarks get messed up and I can't load URLs from the address line. I had previously uninstalled jzip. What these outfits do should be a crime.

Create a new profile as a test to check if your current profile is causing the problems.

See Basic Troubleshooting: Make a new profile:

• https://support.mozilla.com/kb/Basic+Troubleshooting#w_8-make-a-new-profile

There may be extensions and plugins installed by default in a new profile, so check that in "Tools > Add-ons > Extensions & Plugins" in case there are still problems.

If that new profile works then you can transfer some files from the old profile to that new profile (be careful not to copy corrupted files)

See:

• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Creating a new profile fixed the problems caused by jzip.

07-05-11 When I turned on my laptop (Vista) this morning, I got a Start in Safe Mode and Repair message so I did. It suggested a System Restore, I hit OK. When it started like normal, I waited a few minutes then TRIED to launch Firefox 5.0 but kept getting a Mozilla Crash Report dialogue. So I instead opened Firefox 4.0b12 --error free-- so I could get here to get help. When I tried the Run... firefox.exe -profilemanager step, I get the subject error. When I try to open FF 5.0 in Safe Mode by holding down the shift key simultaneously, I get the subject error and no launch (though no Crash box either). I will try to let the FF 5.0 Updater run when it appears again but it never lets me choose when to Ask Again so I don't think it'll show again until tomorrow. Sigh. Oh, and like someone at a different version of this error blog re WinVista 32bit, now I'm getting the issue of Enter not executing the URL typed in the address bar; I had to click the arrow.

I hope the Update is to fix this problem.