Windows Vista: Installation Problem, program installation gone bad

Solution to your problem.

Clean installation is the solution to your problem. Here is how you can do this:

Some problems with Firefox are caused by a corrupt installation folder, by old files in the location folder, or by extra installed components. Performing a clean installation is a quick way to rule out these factors. All user data is stored in the profile folder, so performing a clean installation will not cause lost data.

Close Firefox completely
Rename the Firefox installation folder or move it elsewhere.
Windows: This is usually C:\Program Files\Mozilla Firefox. "Mozilla Firefox" can be renamed to "Old Firefox"
Mac: This is usually Macintosh HD/Applications/Firefox.app. Macintosh HD/Applications can be opened in Finder, and Firefox can be dragged to Trash.
Linux: This varies based on distribution and installation method. Firefox as distributed with Linux distributions usually does not have a single install folder. For this reason, installing a fresh copy of Firefox is preferred
Download Firefox again from www.mozilla.com and install. (Install instructions for Windows, Mac?, Linux )
If this fixes the problem, you can optionally check the old install folder for global extensions, binary xpcom components, or corrupt/locked files. You can have a user send a compressed corrupted install folder for further investigation, if the user wants to know what caused the problem.
Checking the old install folder is optional; it is usually done when we want to investigate a common issue.

Try a new Firefox profile

Sometimes, the easiest way to get started with an issue that persists with Safe Mode is to create a new Firefox profile. The profile folder contains both extensions and numerous data files which can cause some issues; trying a new profile allows the issue to be narrowed down.

Instructions for creating a new profile are at Managing profiles