All the extensions I use are usable on all versions of Firefox, I don't even look at the extensions that do not support later/earlier versions.
Not really sure what you are saying about the saved webpage issue. I save pages all the time, they always work. I just did a search on CF.com and saved the results. Cleared my browser cache, opened the page, and it showed me my results. Works perfect, just like IE or Opera would do.
As far as browser compatibility, that is an issue with any alternative browser. It comes down to what you use that requires the browser. I very rarely have to use IE with what I do, I can even Windows Update within FF now. There are a few webpages that look odd/won't display in FF, but they tend to be few and far between.
You're absolutely right, it has alot of issues that need fixing and eventually will see as much exploitation as IE or any large browser. But in terms of speed, customization, and user friendliness FF exceeds any other browser I've ever tried.