CSS problems with IE

Have spent the last three nights fixing up the css on my site, as it had become straggly and messy. For some reason I only checked it in Firefox as I went along.

I switched to IE to login to this support forum (as I can’t seem to login via Firefox) and checked my site. It’s truly horrible.

I’m not sure if I’ve missed something that can be easily fixed, or whether I should start all over again – this time comparing each tweak in BOTH browsers? Oh dear. It’s almost 5am. Incoherence strikes again….
:o(
https://www.miaowthecat.com m i a o w