If you liked Microsoft's Developer Toolbar you will love the Firefox version. You can download this toolbar here.
It can do all things that you can do with the MS equivalent but also a lot more (the image above just shows a few options). I especially like the Edit CSS and the edit HTML functionality in which you can edit the page's source and stylesheet realtime in Firefox. Great stuff, check it out.
While we're at it, there is another very nice extension for Firefox. For me, the only problem I have with Firefox is in fact Internet Explorer. Some website developers still think that there is only one browser in this world and you can see that very clearly in the sites they build. For this, I now have the IE Tab extension. With one click you can load the page in an IE instance within Firefox. You can also add pages or domains to a list that will load in an IE instance by default. Also a must have.