I don't recommend uninstalling.

close all windows of firefox, and give it a few minutes in case it's saving anything from your last session. If you still get that error, you can hit CTRL+ALT+DEL and see if there is a process called firefox running, and if so, stop that process.

Alternatively, you might consider restarting your PC.

I offer that advice with no warranty as to how good it is. Killing processes is generally not something you want to do unless you have to, but I've run into the same issue and had to do so to solve it.

your suggestion worked but I also had to remove firefox.exe*32 from processes. There were two files. It then installed properly but will not launch. I uninstalled again with the close program message and found those firefox.exe*32 were the only processes or apps open. Deleted them and uninstalled. Reinstalled with no problems and it still won't launch. I am so frustrated.