You can use Foxmarks with FireFox for all those valuable bookmarks - i
find it works very well. As for emails to some extent it depends how
you've set up your system: if you use imap then you simply install
Thunderbird again and it downloads all emails again, if you use pop3
and you mark the box to leave emails on server then you can still do
the same, however if you have downloaded all emails and have nothing
left on the server you can use ImportExportTools to save the folder to
html or mbox.

I use imap and then Importexport tools to archive my emails. hope this helps.

https://nic-nac-project.org/~kaosmos/mboximport-en.html