Thanks very much for the hard work Jerry. I appreciate this is still a beta and there will be a few problems to iron out.
I now have syncing to Firefox and Chrome operational (I had to uninstall and re-install the Firefox plugin). I had to do a lot of fiddling to get notifications working - BkmxNotifier-B didn't appear in Notification Centre settings, although A was there. After giving BkmxAgent full disk access and a full reboot it finally appeared.
However I now have two problems:
1. I have my collection set to automatically open when I start BookMacster. However, instead I get error 523499 when I start the app. The collection doesn't open and I have to manually open it and set up syncing again. The collection is stored in a folder on DropBox. This was working fine in version 3.0.
[Edit]: Also reported here:
https://sheepsystems.com/discuss/YaBB.pl?num=16632016752. When BookMacster does a sync is deletes all bookmarks in each browser and re-writes the entire collection (maybe a result of the issue above?). This works OK in Safari and Firefox. But in Chrome it writes the full set of bookmarks but then adds a random selection of duplicate folders to the top level - I have all my bookmarks in the Bookmarks Bar, so I get the normal set of folders and then about 30 additional folders randomly picked from my collection. Each time I do a sync the set of additional duplicates changes - it seems totally random which ones get added.
NOTE: it might be worth adding a warning for users who, like me, sync bookmarks across multiple computers via a cloud service, to avoid the collection file (and backup) being corrupted during the upgrade of the file storage format. Is DropBox still a good choice for this?