Hello, all is explained here -> https://github.com/aaFn/Bookmark-search-plus-2/wiki/Permissions-and-Privacy-policy

The 2 reasons for a bigger amount of storage than 5 MB are because:
- Firefox does not give access to favicons, so they have to be stored in local storage https://bugzilla.mozilla.org/show_bug.cgi?id=1315616 and https://bugzilla.mozilla.org/show_bug.cgi?id=1411120
- Firefox is very slow on the query to get all bookmarks and so to be fast on next starts after 1st run, the add-on has to cache them in local storage https://bugzilla.mozilla.org/show_bug.cgi?id=1396364

and some users have very large amouns of bookmarks (> 80000), so 5 MB is not enough.
By the way, if you have any doubt, the code is plain open and FULLY COMMENTED FOR READABILITY, anybody can read and look at it to verify.

Thank you for your understanding, aaFn.