Have you heard of Ibis Reader? It’s a browser based reading app that will let you open and read an Epub from any browser.
But it has one minor problem; you can’t read your own eBooks, just one you download from a limited number of sources.
I found a couple Firefox plugins that can read your Epub: Lucifox and EpubReader. Normally I’d discuss the qualities of both plug-ins, but I cannot get Lucifox to run on Firefox v4.x. EpubReader worked without a problem, so I strongly urge you to use that one. EpubReader supports TOC, 2 columns of text, and in general is a very nice reading app.
There are a lot of very good reading apps available, but I prefer to do more in a browser rather than run a bunch of apps. Part of this is because my old computer was slow, but I’ve also noticed that I can switch between tabs in Firefox faster than I can switch between apps. This is part of the reason i don’t use a Twitter app and instead visit the website from my browser.
Update: The crossed out section was a factual error. Thanks, Liza.