Added 4/24/2020:
After living with the outdated Firefox Ver. 60.xxESR for many months I finally felt I needed to update to a new version in spite of losing easy access to all my bookmark notes. I backed them up and planned on accessing them as an html file in Kompozer.

I then upgraded to the current standard Firefox version and with crossed fingers took another shot at adding BOOKMARK NOTES. Lo and behold it came up correctly and imported all my bookmarks on the first try.

It's been several days now and it is still working properly. As such, at least for now, I'm upgrading my rating to 4 stars. If not for the nasty experience 5 months ago I would have given it 5. If it continues to work problem free over the coming months I'll likely come back and step up my rating to 5 stars.

I don't know why it wouldn't work right before and it now since there is no recent update listed. Perhaps it's a change in something on my PC or in the newest version of Firefox. I'll keep my fingers crossed it continues to work well.

ORIGINAL REVIEW BELOW

Works then doesn't work. Sometimes it will have the Firefox bookmarks showing and then the next time I start Firefox they won't be there.

Sometimes when the bookmarks are showing the notes are there and sometimes the note fields are empty.

There is no rhyme or reason to when it is working correctly and when it isn't.

I've installed and uninstalled various past versions of Firefox in order to try and get it to work consistently and current or past versions, standard or esr, don't seem to make any difference.

When it works its great. I like it better than the embedded notes system in the older Firefox versions. However I've given up on trying to use it. The last straw was after shutting down the PC with the Bookmark Notes working properly last night, and then turning on the PC this morning and finding the notes were all gone again. I had to restart Firefox 60.0.9 esr several times before the bookmarks showed up, and then they didn't have the notes.....again. I did one last upgrade to the current non-esr version and while Bookmark Notes showed the bookmarks there were no notes in the note field......again. That was it. I was done wasting time trying to make this thing work.

I just installed today and have had very little time to experience this add-on, yet. While I'm still *really* steamed about losing the Notes section to Firefox (can you say PISSED?), this add-on seems to provide the tools to re-enter my notes for each website I needed notes for. However, having to re-enter ALL my notes will be very laborious and I undoubtedly will have lost many important ones in the process. Kudos to the developer; he has filled a very important function for those of us that utilize notes extensively. Thanks much.

BTW, I'd have to say to Mozilla, that the now-gone Notes section of the bookmarks was one of the VERY most important feature of Firefox to me. Now that it's officially gone, I may have to re-think what browser I use. Chrome and Internet Explorer (or the current substandard offering from MS) do NOT offer a notes section, so Firefox has happily been my browser of choice. FIX THIS MESS, MOZILLA, AND SOON.