brainfreezer wrote:

something like a custom search engine for a predefined numbers of links?

If I understood correctly, places queries may be what you're looking for. These are special search bookmarks like Recently Bookmarked and Most Visited, that show up as folders with matching search results as the bookmarks inside.
For example, the following bookmark searches all bookmarks for bugzilla and displays the matches in alphabetical order.
Note that you must either restart Firefox or open a new window and close the old one for these bookmarks to start working.

place:queryType=1&sort=1&terms=bugzilla

• https://developer.mozilla.org/en-US/docs/Places_query_URIs

brainfreezer wrote:

Also, is there a way to see a screenshot for each link in the bookmarks list

• https://addons.mozilla.org/firefox/addon/viewmarks/

Thanks! I found out that I might not need such search engine or places queries if the bookmark sorting in the FF bookmarks manager was faster: After every time I drag & drop a link from my 5000+ bookmarks list into one of the 20+ folders, FF does not respond for 15-20 sec, while simultaneously eating up a lot of the machine's 16 RAM. This used to happen even when my bookmarks list was much shorter. If I could solve this issue then manual sorting would be a bearable task. What could be the problem?

I think that's to be expected when you have that many bookmarks. Firefox keeps history and bookmarks in the same database, so you could try clearing your history to make database operations (like moving bookmarks) a little faster.

• Delete browsing, search and download history on Firefox

If you have the same problem with significantly fewer bookmarks (say, half that many), try starting over with a fresh history and bookmarks database:

1. Create a manual backup of your bookmarks.
2. Open your profile folder.
3. Quit Firefox.
4. Delete the places.sqlite file from your profile folder.
5. When you next start Firefox, you may find that your bookmarks have been automatically restored from the latest backup. if not, manually restore the backup you created earlier.

It may be worth trying the add-on places maintenance

• https://addons.mozilla.org/firefox/addon/places-maintenance/