Hello DanC.
yes I too was missing the old Opera function which they removed. I was then using Firefox and the first version of this add-on by Alice0775, before Firefox switched to Web Extensions in FF57 and deactivated XUL for add-ons, which broke the that add-on. So I decided to give a second birth to it (hence the "2"), however there are several restrictions as documented on the add-on description page.

One is that Firefox doesn't give us access to Tags, Keywords, Descriptions .. see https://bugzilla.mozilla.org/show_bug.cgi?id=1225916
Some others are Firefox API slowness when add-on use some functions .. which forced me to implement caching techniques and other features to be able to handle properly large number of bookmarks of some users (up to 80000 bookmarks for now).
And last is that because XUL is not authorized anymore, BSP2 cannot integrate with Firefox UI, it has to provide its own UI, and so this is why you have 2 trees.

Because of that, there are opportunities for Firefox and BSP2 cache to get desynchronized.
Rare, but can happen, especially when FF doesn't terminate well. For such cases, you will find a "Reload all bookmarks from FF now" button in the options page. I advise that in your case, you press it to resync things.

If you're worried about such desynchronization events more than slowness, or if you are fortunate to not have a lot of bookmarks and then things are quick enough, you can also enable the option "Revert to old behavior of loading full bookmark tree from Firefox API at each restart". BSP2 will be slowlier at each restart, but there won't be any desynchronization.

i hope this helps, else please open an issue on https://github.com/aaFn/Bookmark-search-plus-2/issues. Thanks, aaFn.