Great Idea. Needs More Work. Rated 2 out of 5 stars
Worked on some add-ons, and completely ruined others.
I have two pcs with the same versions of Firefox, and the same add-ons. I exported the settings of PC1, and imported them in PC2. Some add-ons like "Save images to folder" copied the settings perfectly, but others, like "Smartest Bokkmark Bar," did not, and had some of it's settings randomly changed. e.g. My bookmarks bar was moved to the very top, though that setting wasn't chosen in either of the pcs to begin with. Also, in PC1, I had my "SimilarSites" toolbar unchecked so it wouldn't take up space at the top of my screen. After importing on PC2, it was now visible, and I had to get rid of it. Those are just two examples of many possible examples.
Now I'm manually comparing all add-on settings on both pcs to see what else got messed up because I thought I backed up my preferences on PC2 before doing this, but I don't see them saved anywhere. This blows. It's gonna take a lot of time to check everything.