Well, we all know how much Microsoft hates Firefox, but the level of pettiness can still  surprise. Why continue hosting a Windows Live Writer “Blog This” extension for Firefox, if you haven’t bothered updating it since Firefox 2?
So I updated it myself, to work through Firefox 4+, and posted it here for anyone who wants it. Just right click and save.
Please note that if you receive the message that this extension “will not be installed because it does not provide secure updates,” it gets a tad more complicated.

First, you’ll have to follow these steps outlined by Cybernet News:
Mozilla is trying to beef up extension security … by ensuring that the updates are provided by a secure address …. there could be problems when venturing outside of the [Mozilla] site … but
There is a hidden setting that you can adjust … Here’s what you have to do:

1. Start Firefox.
2. In the Address Bar type “about:config” and press Enter.
3. Right-Click and select New->Boolean.
4. A box requesting the Preference Name will popup and you should enter “extensions.checkUpdateSecurity” (without the quotes). Press OK to continue.
5. Select “False” from the box and press OK again.

Once this is done, install the updated Windows Live Writer extension normally, and you should have a Firefox toolbar button like this:

After which, you can delete the security override just created if you like.
But this begs a different question, since you don’t know me … how secure is this updated extension?
Well, if you unzip it (you’ll have to change .xpi to .zip first), then compare it to the original from the  Microsoft site, you’ll find the sole difference is the the Firefox versions specificed in the install.rdf file.
It’s a bit of a bother, but it restores LiveWriter functionality to Firefox.