I followed the documentation in https://developer.mozilla.org/En/Developer_Guide/Source_Code/Mercurial and downloaded FF source with:
hg clone http://hg.mozilla.
Once you have cloned the repo, you have everything: you can then hg up branchname or hg up tagname to update your working copy.
hg up branchname
hg up tagname
UP: hg up is a shortcut of hg update, which also has hg checkout alias for people with git habits.
hg up
hg update
hg checkout
git