saving .bz2 files

[david]

Greetings,

I've noticed that often when I right click and "save as" a file ending
in .bz2, that the firefox downloader (?) adds an extra ".tar"  on the
end. If missed, the file then won't open in Archive Manager unless I
rename it and remove the extra .tar. I understand this bit, what I don't
understand is why the extra .tar gets added in the first place.
Is this a common thing ?

regards

nux