Install firefox. Delete some files from the installation directory (say the classic.jar or browser.jar etc.). Launch firefox. No errors are given. Firefox runs in memory without a window/visible indication. Preferred behaviour: Firefox should allow the user to repair the installation.

How exactly should it repair the installation if you delete files necessary to browser operation?

There is no way for the installer to do this without at least the app triggering it. For example your steps of: Install firefox. Delete some files from the installation directory (say the classic.jar or browser.jar etc.). Launch firefox. No errors are given. Firefox runs in memory without a window/visible indication. would require the app to trigger a repair. MSI based installers can provide this functionality but the standard exe installer won't.

Adding dependency on implementing an MSI

Repair is working just fine in a MSI packages, created using my patch from bug 231062. If a missing file is detected, we can just invoke msiexec with correct options. If the msi is in the system cache, it'll be enough. If the msi is not cached, we can offer to download one. AFAICT, msi 2.0 and later will correctly prompt for admin pass, if invoked by LUA.

We would get this for free just by using MSI correctly, no need for a separate bug.