firefoxPackages: support building with firefox 65

Firefox >=65 will depend on icu >=63. All the older firefox versions
(and derived packages) seem to work fine with this change.

Also the system path environment patch will fail to apply since there
was a trivial whitespace change in the source file. By adding `-l` to
patch we can avoid having to track two patches that do basically the
same. Having patchFlags per file without resorting to pre-/postPatch
would be nicer but there doesn't seem to be a facility for that right
now.