Are you fully up to date with both Windows Updates and Firefox updates?
You can't remove IE7 as it's an integral part of Vista. Vista allows IE7 to relinquish control, but I guess you didn't check out Mozilla's website:
Mozilla Firefox's main page has this to say:
Windows Vista Support: Windows Vista is generally supported with the following caveats.
Windows & Windows Vista
If the browser is installed in a non-default location, software updates may fail. You can reinstall the software to the default location, or to a non-default location as long as the install folder is named "Mozilla Firefox". Alternatively, you can start Firefox by right-clicking on its icon and selecting "Run as administrator" from the menu.
When you launch the browser after it has downloaded a software update, you may get a dialog asking to permit "updater.exe" to run. If you do not allow this operation, the software update will fail.
Firefox can not yet be set as the default browser on Vista. See this bug for more information*.
When installing as a restricted-access user on a shared machine into a location that you can write to, there may still be negative side effects (default browser/other keys not being set correctly). When installing as a restricted access user do not attempt to install over an existing installation in a restricted-access/shared location as this may destroy that installation.
Firefox may hang when closing after viewing a PDF file in some older versions of the Adobe Acrobat Reader plug in. If you experience this, make sure you are using the newest version of the plug in.
The Update directory on Windows Vista is located under the user's directory, in AppData\Roaming\Mozilla\Firefox\Updates\.
Other known issues with Windows Vista can be found in the Vista tracking bug.
Firefox Main Page: http://www.mozilla.com/en-US/firefox...senotes/#vista