This is a good idea and would make Kareha-boards much smoother to read.
I've used bbs2chreader for some time now, and it shouldn't be to hard to create something like that for browsing Kareha-boards.
Actually, I think we Anonymous users should be able to do some work ourselves. I've thought about modifying bbs2chreader to work with Kareha, but since I know absolutely nothing about making Firefox-plugins, it hasn't progressed at all.
I love love the idea of gathering a team of programmers and create applications for use with Kareha.
Making an extension from scratch is tricky, but modifying an existing one is easy: The .xpi file is really just a zip file, that usually contains a metadata file and a .jar file, which once again is just a zip file. Inside that, you'll usually find the javascript source for the extension, which is easy enough to hack.