FF and Chrome seem to be pretty straight forward for the most part.
IE and Safari though I have no idea at all.
I have done FF, Chrome is pretty by looking at the specs.
IE is a fucking whore.
IE is basically using COM/OLE.
But there are 2 parts to an extension.
A "BHO" or browser helper object. And a toolbar. They are separate.
Also they are "compiled" which is another bitch. And when the fucking thing crashes it is hard to find the problem.
Also I think because windoze 64 systems now ship with 2 versions of IE (32 and 64 bit), you would need to compile to sets of extensions 32 and 64.
Since you can not load a 32 bit extension into a 64 bit and vice versa.
Lots of fucking fun.