There's a Firefox extension that allows you to "Blog This to Windows Live Writer". It works very well with Microsoft's unequalled blog authoring tool, I've been using it for months now, but these usage instructions are handy:
The Blog This gesture is available in two places: the toolbar button, and the right-click menu. I tried to be a little smart about what you are trying to blog. If you have a selection active in the browser, it’ll be used for the contents of your post. If you right-click on an image or link and select Blog This, that’s what’ll be used.
The "announcement" post this came from was reposted in error today -- the original came out last year. Even so, it's a good reminder of this terrific extension. The odd thing is that Microsoft offers it at all -- WLW is so good it could have forced me (very reluctantly) to switch to IE . On the Mac WLW is threatening to force me to run FF/WLW within an Win2K VM!
Here's the download link for the released version. As I've noted before I used to have problems making it work, but they went away ...
 I only have two nits with WLW. One is that it's a modern .NET app, so it's slow to launch on all but the fastest machines and uses a lot of memory. Secondly, it quits after use, or stays open with the post remaining. I'd rather after each post it stayed resident but closed the post window -- in part because it's so slow to launch. (Maybe that's one nit?)