Not too much, speaking of MicroB. It'll require more than 'mad skillz' to update it without having the closed-source core bits. You'll have to stick with Firefox Fennec or Opera. Last one is closed source but it has the quickest interval between updates. I dont list Chromium because is almost unusable, like Firefox Fennec that is REALLY REALLY slow.
I see Opera as the best browser for everyday use in Maemo (even if it's closed-source), and the most up-to-date one. I really doubt there are vulnerabilities for it. Sad thing is you'll be forced to use MicroB if you want Flash.
Another alternative is to use browsers in Easy Debian (wich I don't tested too much).