This tip may or may not be relevant for FireFox or more recent versions of Mozilla.
I have a long, hierarchical and mostly unsorted list of bookmarks used by my installation of the Mozilla 1.7.8 browser.
There is a search box, which allows me to locate a bookmark using text matching.
However, if I want to add a new bookmark and have to find the appropriate folder to which add it, the search box is useless for this purpose. Even if it finds me other bookmarks filed in the right folder (based upon their subject), it does not show me in which folder those bookmarks are.
The following hack solves the problem. The directory names are correct for Debian Sarge. If you use another Linux distribution or another OS, YMMV.
Look for the bookmarks.html file in your ~/.mozilla directory, load it (as a regular HTML file) in your browser, and bookmark it!
Its URL will typically be: file:///home/yourusername/.mozilla/default/something.slt/bookmarks.html
Next time you need to search for the folder where to add a new bookmark, load the bookmarked bookmarks.html file and run standard text search (Edit/Find in this page; usually bound to CTRL-F).
Then you can happily add your new bookmark to the best folder for your purposes.