Build: Try to unpack Firefox ESR via xz, fall back to bzip2HEADmain

The `-j` switch passed to `tar` indicates the archive is compressed using the
bzip2 format (`bz2` extension). That was how Firefox used to be compressed until
recently, but the new ESR release now uses xz. Ubuntu `tar` doesn't auto-guess
the encryption algorithm, so to support both, first try with xz and fall back
to bzip2 if that fails.

Note: this will download the old Firefox ESR twice, but it will still work
when the current ESR version starts to use xz with no changes to the code.

Closes gh-5682
Ref gh-5684

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