Problematic if resizing... Rated 3 out of 5 stars
... in more than only one opened tab.
If you have 2 tabs opened and there are *several* text areas on *each* tab, I often find that *some* of those textareas are fixed (won't resize with your extension).
Go to mozilla addons developer tools, choose e.g. edit addon, go to localization page.
As you know, there are several languages to choose from, each language provides several analogous textareas.
Edit textareas on a minimum of 2 tabs (=2 languages).
Try to rezize all areas on both tabs, don't forget to switch tabs several times.
You will find some areas locked (randomly).
This is not the only site, meanwhile I found more sites on other places.
Note: Is this extension still actively developed?