it seems you managed to use linux for 5 years without a single problem? wow.
may i ask what distro you are using and whether it is up-to-date?
without further information we cannot offer much help.
you can watch thunderbirds status bar, maybe get some extra information? or start thunderbird from a terminal, see what it spits out?
anyhow it seems the issue is more general; internet slows to a crawl.
(funny, i'm having similar problems today, but not with all sites)
are you aware what is accessing the internet? maybe some app went haywire.