I must say that even though most websites are designed for IE, it's a rare site I find that doesn't work pretty much the same on Mozilla ... However, that's certainly not a 100% thing. I find that occasionally some javascript functions don't work like the programmers expect them to (usually doing funky things with opening windows -- I think sometimes it's caught by Mozilla's popup blocker).
For all the work they did, it really does seem like IE is faster than Mozilla. I blame XUL for that for the most part. An interpreted javascript browser just isn't the recipe for lightning quickness
However, it is really cool that the source is sitting there, so it's pretty easy to add in new features (like I added in opera-style google searching from the url bar "g <searchterm>" to mine).
As far as font sizes, I must have modified my settings, because it looks OK for me size-wise. You're right that the caret position is sort of a funky choice. I'm not sure why they chose that, though the one or two pixel overlap doesn't really bother me (I'm too blind to see it @ 1600x1200 on a 19" monitor anyway).
I stuck with IE for a really long time ... Finally I just tried the switch and I've really been pleased with Mozilla. The main thing keeping me in Mozilla is the amazing multizilla tabbed interface controls ... Everything you ever wanted to do with tabs and more. It totally changed the way I used my browser having them ... Also popup blocking is pretty damn nice, though I guess you can get that w/ the google toolbar (haven't tried it to compare effectiveness).
ms