i have no problems with Firefox on Win 8.1 and if you upgraded to Win 8.1 from Win 8 and had Firefox on the pc this could be
part of the trouble
after uninstalling Firefox did you clean up the folders left over after the uninstall mabe it will help to delete these
when uninstalling software alot of the time files are left over and can make trouble to reinstall
Uninstalling Firefox
http://kb.mozillazine.org/Uninstalling_Firefox
make sure you uninstalled Firefox and then delete these files if you have not already done so
C:\Program files\Mozilla Firefox or, on 64-bit Windows, C:\Program Files (x86)\Mozilla Firefox
Enable viewing or searching hidden files and folders go to the Control Panel then Folder Options In the Folder Options window,
click the View tab. Under Hidden files and folders, select Show hidden files and folders uncheck the Hide extensions for known
file types box, in order to see the file extensions for all files and the Hide protected operating system files (Recommended)
then look here (if you have more than one user accnt.on the pc delete in other user also)
Windows 7/Vista: C:\Users\<username>\AppData\Local\Mozilla\Firefox <---- Win 8/8.1 should be the same
Windows 7/Vista: The C:\Users\<username>\AppData\Local\VirtualStore\Program Files\Mozilla Firefox folder (if it exists)
after deleting what Firefox folders you find go back to the Folder Options in view click Restore defalts and apply
after you delete this if you use Ccleaner you can run it then use the registry cleaner part of it to also get left overs in the
registry (but take care running registry cleaners can mess up your pc) i have not had trouble ever using Ccleaner's registry
cleaner but you should know that messing with the registry can make trouble
now try to install Firefox or Pale Moon and see if this helps
James