Euuuuh, Firefox is open source, why try to decompile the executable?
The licenses: MPL, GPL, LGPL. See what you can do with them.
Finally, taking the program from there and doing whatever you want with it already requires a good level :-|
The terms you're looking for are indeed disassembling and decompiling, but the two are different :)
We disassemble to a low-level language and decompile to a high-level one.
See:
https://fr.wikipedia.org/wiki/D%C3%A9compilation --
The urgent is done, the impossible is in progress,
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