In the past, I have used del.icio.us, Google Bookmarks and a host of other service to sync my bookmarks between installations of Firefox. In the end, I always quit using them pretty quickly (well, for synchronization at least, I'm still using Google Bookmarks for other stuff). I usually quit them because in the end, they didn't work. For example, del.icio.us (and Google Bookmarks) use tags to order your bookmarks. By itself pretty cool and very 21st century. Firefox however doesn't have a very useful interface for using a lot of tags. I have nearly 200 tags registered in my Google Bookmarks account. No problem for a web-based service like Google Bookmarks, but not very useful when you have a single column bookmarks menu in Firefox. I now tend to use Google Bookmarks for sites I need to be able to find again or sites that are an interesting read for later on. I still use the bookmarks menu in Firefox as my default bookmarking service.
Enter Weave. Weave is a Firefox extension / service from Mozilla Labs that lets you sync bookmarks, history etc. between Firefox installations. It needs Firefox 3.1b3 at the moment, so it's pretty bleeding edge. Nice feature is the fact that your stuff is stored encrypted by a passphrase you to supply when registering.
Now, imagine you have a Fennec installation on a small form-factor device. Fennec has Weave too, so I can use my main Firefox bookmarks, history, yes even tabs on the Fennec browser on my N800. I know Opera also has a feature like this, but I've never felt comfortable with Opera, so that just won't do. Besides, there is no Opera for my N800 :-)
So, to make a long story short: keep your eyes open for Weave.