so i saw nice looking windows 7 aero theme on the web and downloaded it and installed windowblinds for it.
but unlike the rest of my apps that chanced immediately as the theme got applied, firefox did absolutely nothing.
so i typed about:restartrequired and did restart of firefox, and to my horror my customized ui like tabs not on top and other similar stuff was gone.
why is that? because your software didn't bother to think that if user has chrome folder, userchrome.css file inside that folder and firefox is set to use said css file by setting about:config -> toolkit.legacyUserProfileCustomizations.stylesheets -> thrue
that maybe they already had some settings there that they WANTED TO KEEP!!!
lucky for you that windows file backup utility was able to restore recent enough version of that userchrome.css file that firefox started to work again.
how hard it is to make that tool to check if user already has chrome folder inside C\Users\*****\AppData\Roaming\Mozilla\Firefox\Profiles\
then create at least a copy of the userchrome file that you are in the process of ruining. such check should not be that hard.
needless to say, that i'm not going to be purchasing a license for this app after such experience, but this kind of check is such basic thing compared to the rest of this app that this should be added soon as possible.