I believe, that as a Blog-er, I would be lost without Firefox. Firefox has so much to offer the Blog-er. Tabs alone would make this a far superior Blogging tool. But it appears as though all major browsers will now have tabbed browsing. So it comes down to the extensions.
this list is not in order of importance. It's just in a numbered format.

1. Tab-mix-plus -- for the little tweaks you can make to how the tabs operate and appear.
2. Context search -- Just for the ease of selecting text and searching it using any search engine you want. (this ties in with being able to add search engines to your search bar)
3. Customize Google -- Saves time when searching and also sometimes gives me something that I hadn't thought about.
4. I.E. tab -- Allows me to see how my slightly customized template appears to those using Internet Explorer.
5. CoLT -- This has become one of my favorites. It allows me to copy a link(URL) then it will turn it into an HTML formatted link so I can paste it into a sidebar. This eliminates a fair ammount of manual coding, which saves me time and makes me happy.

I have a few other extensions, but they don't really pertain to blogging.
Of course there are other features of Firefox that are just as important. Such as the

ability to re-organize the order of the tabs...

the ability to close a tab with a middle click...

the ability to add Digg to my search bar so I can see what others are talking about..

the ability to see a preview of my blog in tabs and quickly compare it to other previews to see what the coding change has done to the appearance.

The password manager (not perfect but it helps)

I can't think of any more. But if I do I may post them.

I am open to any suggestions you may have for extensions that enhance your Blogging experience. Please comment...