I think the Firefox problems are common and nothing to worry about.
The DNS lookup slowness has happened to me with previous ISPs - a slow DNS server that's even slower in peak times - I could sometimes wait 30 secs for a lookup. You might want to look at bandwidth issues, connection quality and your ISP's/DNS service reliability. Perhaps using OpenDNS for a while would give you something to compare.
As for Kmail - I don't use it and I haven't seen a problem like that before, but it strikes me as something a hacker/cracker wouldn't bother with, so more likely to be a configuration error or similar (maybe you accidentally toggled read/un-read recently?).
Several router models I know don't show incoming traffic, only outgoing connections and firewall block events, so depending on what application you're using to view router data (which I assume is syslog-style data) the fields may be empty not because of tampering but because your router doesn't record that data.
There doesn't seem to be anything to suggest you've been hacked/cracked, IMHO.
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It's official: people are idiots.