https://chrome.google.com/webstore/detail/waxpost-weasyl-crossposte/boddplgmdhddfmchbifhpdpekfdaenjc I haven't used it myself, no firefox version still~

Yes I have this! Just follow the link Noiserut gave and install. Then any time you go to an FA, Deviantart, or SoFurry submission (it even works for submissions that aren't yours??? which is kind of weird but hey whatever), the Weasyl icon will show up inside the URL bar, all the way to the left. Click that and it will automatically open the submission page on Weasyl, with the file, description, title and tags taken straight from the original. All you have to do is add the category, folder, and rating (and modify the tags sometimes, since Weasyl's system is little different than most sites). It's pretty awesome, honestly. Saves a lot of time if you post to multiple sites. I just wish it worked for Journals too lol

While not a crossposter, the Simple Ottermatic Follower Transfer might also interest you. With it, you can import your watchlist from FA to Weasyl.

You could just use Chrome for this one thing and then not use it anymore?

One of these days, I'm going to get off my butt and make a Firefox version. Because I want it, too! And maybe I can make it post to Tumblr, too...

there might be a firefox extention soon, i just gave the teck team sone help in creating one