I need to share some bookmarks. I have given them all a different tag in Firefox [3.5.3]
Now How can I export only the bookmarks under that tag so that they can be shared?
There's no way to do it natively through FF. You'll either have to find an extension that will do it or export/backup and edit the resulting html/json files.
So, what you can actually do is use a second browser that you don't use but have installed, or quickly install one you don't, such as Internet Explorer. You would want to then go to the bookmarks manager of the browser that has the folder or tagged bookmarks, then select all of the relevant bookmarks and copy them by right-clicking and selecting copy. Then you can paste them into the secondary browser's bookmark manager. At this point you will only have the bookmarks you want on that browser, at which point you can export the bookmarks from that browser as usual via HTML, and easily share it wherever you want. Sorry for being so late to answer.