firefox: Remove hardcoded path from desktop entry

A common pattern to write wrappers for applications (e.g.: to run Firefox inside a sandbox), is to place an executable with the same name in /usr/local/bin/ or ~/.local/bin/ or any other path with higher precedence in $PATH.

Firefox's desktop entry is one of the very few that includes an absolute path to the binary. This has a slight downside: it doesn't use wrappers which follow the above pattern. OTOH, using an absolute path doesn't seem to bring any upsides (I suspect this approach exists mostly due to historical reasons).

The safe-mode desktop entry remains unchanged, since that one does not have a hardcoded path.