Show File Size for Firefox shows you file sizes before you download
Show File Size is a tiny add-on that does exactly one thing: it adds the file size to Firefox's file download dialog.
In true Firefox style, even this one trick pony has an Options dialog: It lets you set how many decimal points you want to round the number down to. So, do you want 5.317 MB, or is 5.3 MB more your style?
Joking aside, though, this can be handy for some situations. Most serious websites do list the file size next to the download link, but every now and then I click on a link that should lead to a tiny application, and then I find myself downloading a 50MB piece of bloatware.
More than anything, really, I think this is yet another testament to how customizable Firefox is. It's not a whole new dialog; it's a seamless modification of a very specific part of the program, and it's done in a way that feels completely native. It's nice, and I don't think it's something you could do with Jetpack (right?).