Google-chrome – Google Bookmarks plugin for Chrome

Yes, as jcnaquin suggested, Xmarks is awesome.

(Chrome Extension, Firefox Ad-on)

Not only can you do cross-browser, but you can set up profiles for different computers (Work/Home/Custom) and determine which bookmarks/folders go with each profile. This can help you choose if you want specific bookmarks to be at home, or if you want to leave them at work.

In addition, you can sync Passwords, or Open Tabs on each computer, but only if you want to, of course.

Two options available within Firefox itself:

Go to (in versions < 4) Bookmarks>Organize Bookmarks or (in 4+) Bookmarks>Show All Bookmarks. There will be a button-dropdown for backing up/restoring/importing bookmarks near the top of the dialog box that appears. There will be options to export your bookmarks as either a JSON file or an HTML file. Restoring from the JSON file will completely replace the current bookmarks, while importing the HTML file will just add all the bookmarks to the Bookmarks menu.

I think the HTML version might be better for merging bookmark files, but would require you to delete all the current bookmarks in Firefox before importing the most recent HTML file to avoid all the duplication that would occur. The JSON version does not have that problem, but I'm not sure how well-suited it would be to merging, as it doesn't use line beaks.

UPDATE:

Just noticed that Firefox 4, at least, offers the option to sync bookmarks, etc. Go to Tools>Options and select the "Sync" tab.