SearchPlaces is a Firefox/SeaMonkey extension that allows you to create your own complex Places queries (i.e. bookmark and history searches), or edit the ones that come as standard with Firefox/SeaMonkey, or created using the Bookmarks Organiser.
Queries are viewed just like bookmarks and dynamically display the results of searching the Places database containing all your history and bookmarks. They can be useful to see e.g. which bookmarks you've used the most times, or which contain a certain word. Using SearchPlaces you can put together quite complex queries to meet your specific requirements.
This is one of a number of extensions I've written that you may find useful.
June 21st 2011 - SearchPlaces v1.4.1 released:
June 11th 2011 - SearchPlaces v1.4.0 released:
March 24th 2011 - SearchPlaces v1.3.0 released:
January 17th 2011 - SearchPlaces v1.2.4 released:
January 9th 2011 - SearchPlaces v1.2.3 released:
To begin select the SearchPlaces option in the Bookmarks menu. This will display the query selection dialog:

You can select an existing bookmarks query from the display (use your mouse or keyboard to navigate the tree and highlight the query you want) and edit this using the 'Edit' button. Alternatively, you can choose to create a 'Copy' of the selected query and edit this to create your own version. Or you can create your own query from scratch using the 'New' button. In each case the query editor is then displayed:

At the top of this window is the name of the query, which will be blank for new queries and can be modified if you which to change the name of an existing query. Below this is the URI of the Places query itself, as used by Firefox/SeaMonkey. You can directly edit the URI if you know what you are doing, and 'OK' to save this, but most people will use the options on the 'Parameters' and 'Attributes' tabs to create or edit this URI.
Click on the 'Apply' button to apply any changes you've made to the options to update the URI. Click on 'OK' to apply the options (unless you've manually changed the URI) and save your changes. Note that new queries will be automatically put into your unfiled/unsorted bookmarks folder. You can then move it from there to wherever you wish to place it using the Bookmarks Organiser
Full details of what all these options mean is outside the scope of my knowledge, but I took them all from this Mozilla page: https://developer.mozilla.org/En/Places_query_URIs
Note that I've found that some of these options appear to be incompatible with each other, and many are pointless in terms of the results, e.g. folders are never displayed, except in the extreme case noted below, rendering some options useless. Also some of the options haven't been fully implemented yet. Personally I've found it difficult to work out how and when to use the 'Folder to query' options and the 'Query type', which appear to be interlinked. Trial and error with the Bookmarks Organiser open at the query often appears to be the best approach. Remember these are just queries and so shouldn't damage anything. So don't contact me if you get no results from your queries, contact Mozilla for help instead. Of course do let me know if the Places URI isn't being generated or parsed correctly or there are other issues with the editor and I'll fix them.
Additional note: If you select, say, 'Bookmarks menu', and don't specify any other options, then your query will pretend to be the 'Bookmarks menu' itself, and all folders and bookmarks within it will be displayed. Similarly for most of the other 'Folder to query' options. However, you will still be able to select and edit this query even though it will appear as a regular folder in the query selection dialog.