Since bug 16707 resolved, MediaWiki sets rel="next" to the returnto. Although the stated reason is for UX (e.g. it mentions Opera can show some kind of Next link/button), it has other impacts.
Firefox (and possibly other browsers) use this as a pre-fetching hint. For regular pages, this can be fine. However, some special pages (I know of ConfirmEmail and InvalidateEmail, but there may be others) take an action on a regular GET request, even though this isn't generally best practice.
Firefox will not pre-fetch if there is a query string (see https://bugzilla.mozilla.org/show_bug.cgi?id=175418), so my proposed fix adds this to the rel=next URL, only for special pages that don't already have a query string. It does not affect the actual displayed returnto link (<a> element).