Rated 5 out of 5 stars

Find a way to install it... (working on version 60.0b11 (32-bit)
Donwload the file mark_all_read_button-0.7.1-tb
Change extension to zip
Unzip the file.
find the file install.rdf
Edit install.rdf replace ligne 27 <em:maxVersion>31.*</em:maxVersion>
save the new install.rdf
Zip again all the files
install it... Good luck...

This review is for a previous version of the add-on (0.7.1).  This user has a previous review of this add-on.