Firefox problems when remapping Copy to ctrl-c
You can use a tool like BetterTouchTool (or some similar shortcut remapping program for Mac) to remap Control C to Command C. This will prevent Firefox from knowing that you pressed Control C at all, and thus it won't enable the "Match Case" option in the find dialog box.

Alternatively if you are interested you can read up the documentation on Karabiner Element's new complex modifications feature to remap Control C to Command C in Firefox. This is similar to doing it with any other program, although I suspect that it is more reliable. However, it requires a bit more knowledge of how to add modifications to Karabiner Elements, because (as of writing this answer) the feature is quite new to Karabiner Elements, and there is no GUI method of creating complex modifications (apart from importing one from the website, but you might not find the one you are looking for).