We have already seen it before and I will show you again:
In the case of Mail Client, I was testing version 13 that integrates this option, and at least for now it is very limited, since it simply allows using the accounts of GTalk, Facebook, IRC y Yahoo if i'm not mistaken, so any other service like XMPP it is ruled out unless it can be integrated through extensions.
The point I want to get to is which one do I use then for the Instant messaging? If you ask me, I only have one answer to give: None. From the outset, I do not see how adding these "extra" functionalities to both applications can improve their performance. If already, so much Thunderbird as Firefox consume a lot of memory, I don't want to see when they integrate a IM client. Unless they somehow manage to optimize resources even more, which I doubt.
The idea of integration does not seem bad to me, on the contrary, it can be very productive. A case of success would be, for example, if we check the Email online (Gmail, Yahoo ... etc) and therefore we only have to open the browser, so it would be very useful to have the IM client available. But in my case, I usually have both applications open the whole day, and I am one of those who prefer that they only do what they have to do, Firefox to navigate, Thunderbird for the mail. For messaging I have to Pidgin, which supports how much protocol exists and that function is 100% fulfilled.
I prefer developers of Mozilla spend your time working on the consumption of both applications, improving their appearances, and the basic options, than trying to implement something for which we already have a few alternatives. That is my opinion. What do you think?