Comment 26 for bug 1981163

The workarounds are not helpful if you a) want to view pdfs in an external viewer like acroread *and* b) want to view html attachments in Firefox. That would only work if FF itself opens pdfs in the external viewer (but means the pdf is saved twice). Better workaround would be a little shell script as helper for html attachments in TB which will determine the filetype and send it to either FF or e.g. acroread. One-liner in Linux, sth. like `/usr/bin/file "$@" |/usr/bin/grep -q -i pdf && /usr/bin/okular "$@" || /usr/bin/firefox "$@"`

Unfortunately, it seems that devs are ignoring this bug so far as it has not been assigned to anyone.