It works even with FF 13 Rated 5 out of 5 stars
You can make any add-on being accepted by FF easily . Go into about:config and add for your FF version (mine is 13.0) the entry
This is a boolean value. Set it to false.
If you upgrade to a higher FF version just add another entry with the new version information.
For me the Line Marker is a great add-on. I use it with the great add-on UnMHT where you can save webpages or a selection out of it in the compressed mht form. The marked text will be saved as marked with it.
Rated 5 out of 5 stars
Perfect. I save a lot of web pages as MHT to keep the information. Line Marker helps me to point to the important parts in all the saved text.
For me it works without any problems even with Firefox 3.5. A great tool - thank you for it!!! Please make an update of the allowed Firefox version so that more people can use it.