Hi friends of Thunderbird and connoisseurs of "URLs for everything" ;-) URLs in Thunderbird have had a very tough time recently, since M-C made the underlying class nsIURI immutable, that means all setter methods were withdrawn. If you want to get a URL with a different attribute, you need to make a clone and in the process of cloning change the attribute. This is now done via so-called mutators and looks like this in C++ NS_MutateURI(url1).SetSpec(aSpec).Finalize(url2); or like this in JS: url2 = url1.mutate().setSpec(aSpec).finalize(); where url1 and url2 can be the same variable. There is a lot of fine print, be let's ignore that here. TB has four URL classes, one of which has many derived classes. The three simple URL classes are nsMailtoUrl, nsLDAPURL and nsAddbookUrl which extend nsIURI and have no derived classes, and nsMsgMailNewsUrl from which we derive SMTP, IMAP, JSAccount, Mailbox, POP and NNTP URLs. nsMsgMailNewsUrl extends nsIURL (not nsIURI). In Thunderbird, "extension" is done via composition[1], that means, the classes carry a member variable, typically called 'm_baseURL' to reference an instance of the class being extended, to with many methods are forwarded, like for example: NS_IMETHODIMP nsAddbookUrl::GetPort(int32_t *aPort) {   return m_baseURL->GetPort(aPort); } Now that the extended classes are immutable, that means that the setter methods look like this: nsresult nsAddbookUrl::SetPort(int32_t aPort) {   return NS_MutateURI(m_baseURL).SetPort(aPort).Finalize(m_baseURL); } So far so good. "But hold on", the astute reader will say, "won't cloning make existing references stale?". The answer is "yes". Take for example: nsCOMPtr<nsIURI> currentURL = url;  // Keep a pointer to the URL we're currently processing. NS_MutateURI(url).SetQuery("number=1").Finalize(url); Oops, currentURL is now stale pointing the previous URL object before the clone. To add insult to injury, Thunderbird's URLs also hold a lot of state information in additional member variables of the object, so if 'url' in the example were to be processed further to adding some listeners, the 'currentURL' wouldn't hear about those changes. To work around the problem, a few "internal" methods were introduced on nsIMsgMailNewsUrl: SetSpecInternal, SetPortInternal and SetQueryInternal. These methods are available to C++ code and allow setting attributes of the underlying nsIURL object by cloning it (see "composition" above), without cloning the nsMsgMailNewsUrl object. Note we easily confuse interfaces (nsIMsgMailNewsUrl) and classes (nsMsgMailNewsUrl) implementing them, and there is of course no "nsIURL object", the class that implements nsIURL in Mozilla is class nsStandardURL, which in the new scheme is created like this: nsCOMPtr<nsIURL> url; NS_MutateURI(NS_STANDARDURLMUTATOR_CONTRACTID).SetSpec(urlstr).Finalize(url); But that is material for another post. The URL work is now complete and Thunderbird still works, as you can see here and by looking at a mostly green tree :-) Jörg. P.S.: The horrible details can be found in these two bugs: https://bugzilla.mozilla.org/show_bug.cgi?id=1431913 https://bugzilla.mozilla.org/show_bug.cgi?id=1440693 P.S.2: Thanks to Kent for interrupting his retirement to take care of JSAccount URLs. Kent, please try ExQuilla now! [1] https://en.wikipedia.org/wiki/Composition_over_inheritance