Could potentially become VERY useful Rated 3 out of 5 stars
Context Peek could be one of those rare "must have" add-ons, maybe even one of the kinds that becomes integrated... However, it has to overcome a couple of flaws first.
1) It really needs to let the user control the size of its window, preferably based on each individual entry since they won't all need the same amount of room. On my system, it was only big enough to show maybe 1/5 of its Translator, and I don't use particularly large fonts.
2) It should have a "test" area for trying out commands and/or implement changes/additions right away, not ignore them until Firefox restarts like it does now. When a person's work doesn't do what's expected, it's pretty rare for them to figure out how to 'fix' it without some trial-and-error -- waiting for Firefox to restart after every attempt to see if the changes worked would be a tedious time-waster.