Rated 5 out of 5 stars
I am posting this review with the approval of the developer, who has kindly agreed to it and has confirmed by e-mail that he's unlikely to continue maintaining the add-on in the future.
HighlightAll provides a unique and very useful feature! It is very easy to use and definitely improves the browsing experience. However, HighlightAll will unfortunately break completely in the upcoming Firefox 27.
I have written my own add-on, called "FindBar Tweak", which amongst its many features, can also function in a very similar way as HighlightAll does. I am also still actively working on it and have planned a few future improvements specific to this functionality as well.
I would invite everyone to switch over to FBT, which ensures it will remain compatible with future versions of Firefox.