[Carnage]

so just leave the id and add the name
domain.com/forums/10-off-topic

Thats the default setting without installing this mod. If you have your forums installed in forums however, setting the forums prefix to be blank will probably do this. (If you get a // in the url set it to http://www.domain.com/forums instead)

will this work with a sitemap?
if not, than I don't find it useful ?

Ive not tested it with the site map; I assume that the sitemap uses the get_friendly_url() function thou, provided it does, then this will work with the sitemap.