This is the best virtual reading ruler I've found. Thank you. It helps me with Visual Snow Syndrome.

I wish I could use it to highlight whole paragraphs though, not just single lines. Haven't been able to find an app that does that. That would be amazing.

So pleased to find this add-on as I've been looking for something just like this to help me with reading content in a second language that uses a different script. It's easy to get overwhelmed with all the text on the page, so having something that helps me focus just on the part I'm reading is really useful.
The one possible improvement would be the ability to set everything else on the page (everything not highlighted) to fade a bit, at the same time, to the background colour of the page. Initially I was looking for something that would give me almost a keyhole view on just the text under the mouse, so that's where that idea came from.
Also, it would be nice to be able to choose for the add-on to be disabled by default on new domains so that using it is "opt-in" rather than "opt-out". Right now I'm having to check the box to disable it on most of the pages I visit frequently (but on which I don't read long-form content). I'll get there in the end, but right now it feels like work and could discourage new users.
But overall am really pleased to have found this and very grateful to the developer.

Works great. Pretty much what I was looking for. Would have, however, loved a universal toggle to quickly switch the Ruler Mode on/off across websites.