This add-on works great! I have been looking for an extension like Xmarks for several years after it was shut down by LastPass and this does the job well. It takes a bit of time to set up, but--once it is running--it is amazing. I am able to easily sync between Firefox (and branches), Chrome, Edge, & Brave across two different profiles. Synchronizing with Opera (because of its different structure for bookmarks) ended up making a bit of a mess. Strangely, on Brave, it took what was on the bookmark in every other browser and put it into a folder with folders for other bookmarks all in the bookmark bar. While that one wasn't perfect, having all my bookmarks everywhere is great. Even the mobile app allows for two profiles, which keeps the phone in sync as well.

It would be even better if they would make an extension that would install into Firefox mobile.