Both the Thunderbird email client as well as the SeaMonkey Internet browser have taken a backseat when it comes to development resource distribution over at Mozilla. The products are clearly not as popular as Firefox, and since resources are limited, Firefox is clearly the one product that is getting the most love on Mozilla.
Development resources were moved from Thunderbird last year, which had the result that all releases for the email client afterwards were more or less limited to security and stability fixes.
Seamonkey users on the other hand may have noticed that their product has vanished from Mozilla's product page recently whereas Thunderbird is still listed on the page.
Add-ons on the other hand are still accessible on Mozilla Add-ons, so that Thunderbird and Seamonkey users can download and install extensions from the official website for their product.
If you are following the development over at Mozilla you may know that the organization plans to merge AMO, that is Mozilla Add-ons, with the Firefox Marketplace.
A very likely consequence of this is that Thunderbird and Seamonkey won't be supported anymore on Mozilla Add-ons. A recent conversation between Philip Chee and Mike Conley, Seamonkey and Thunderbird leads, indicates this in clear terms:
(Philip Chee) Well the reason I ask is that once amo gets merged into marketplace, TB and SM extensions are going to get the boot
This stance is confirmed in a conversation between Philip Chee and Jorge Villalobos, Mozilla Add-ons Developer Relations Lead.
(Jorge Villalobos) It's likely that when marketplace and AMO and consolidated (whenever that is), applications that aren't Firefox will be dropped entirely.
What we know
We know that Mozilla will merge AMO and the Firefox Marketplace in the future. There is no data for that yet, and it may very well be in 2015 the earliest.
A very likely consequence of the new AMO is that Mozilla products that are not Firefox related won't be supported by it.
This means that both Thunderbird and Seamonkey will have to find alternatives that they can use to provide users with extensions for the products.
A likely outcome is that Thunderbird and Seamonkey extensions will be moved to another domain, or two domains, from where they are offered to users. It is highly unlikely that this means the end of extensions for the products, but a solution still has to be found. In regards to that, a solution for existing add-ons needs to be found in the same context.
What is irritating is that both product leads have not been notified by Mozilla about the upcoming changes. (via Sören)