www/firefox-esr: fix rust warnings "variable does not need to be mutable"

This led to build failure in thunderbird on some toolchains,
I'd expect the same here, so fix it.
https://bugzilla.mozilla.org/show_bug.cgi?id=1841919

(cherry picked from commit ea1caa7f0b66163c6144b7a91b7c78a23280dcf9)