As new Thunderbird user I was exploring its UI and noticed (besides rather basic HTML typically used in rich-formatted emails) that add-on manager opens a tab with https://addons.thunderbird.net/ and allows a limited browsing (no back button and address bar) of that website. The rendering of the website sort of implies that I view it thru fully-featured (or limited) web browser. Given Thunderbird is a sister project to Firefox and having a similar architecture, I can come to conclusion what this Thunderbird component is Mozilla Gecko. Could someone please positively (or negatively) confirm that? And maybe add some explanation on the "internal browser's" capabilities and limitations. Unfortunately, Thunderbird is very poorly documented, even developer's article on that subject has not been written yet.

So far this question is purely theoretical, but if it is really fully functional Gecko there could be some uses for it, eg. viewing PDF attachments via built-in pdf.js or even submitting basic subscribe/unsubscribe web-forms. But this is rather broad topic, let it be the separate discussion.

2 Answers 2


Yes, Thunderbird contains Gecko. Gecko isn't actually a browser, it's much of the engine on which a browser is built. It's in all Mozilla products that relate to web browsing and email (and a number of third party products). For a better understanding of what Gecko is and does, see https://developer.mozilla.org/en-US/docs/Gecko/FAQ.

It's listed in the Thunderbird user agent (see this list, and for example, this parsing). It is used as the layout engine for rendering messages and as part of the Thunderbird user interface. As far as browsing the web, Thunderbird uses it to a very limited extent to view a designated collection of pages such as the add-on related stuff. That is a very controlled range of content.

But Thunderbird has no provision for you to specify an arbitrary URL for it to render, and doesn't contain the supporting infrastructure to handle the range of content you can encounter on the web in general. Any links contained in messages are opened by the default browser as specified in your OS settings. See this link and this link. Some step-by-step instructions for setting a Thunderbird-specific browser here.

There are also some add-ons that let you select a different browser to open a link, but you can't specify Thunderbird, itself. The same applies to rendering any other type of attachment.

There used to be an add-on called ThunderBrowse (and perhaps others?), that allowed you to view web links in Thunderbird. But that is no longer compatible with the recent versions of Thunderbird, and I don't know how it worked (i.e., built on top of the internal Gecko engine or just a lightweight browser that got integrated).

The documentation on Gecko is, indeed, a bit hit-or-miss. Many of the available links haven't been well maintained. There's a good general discussion on Wikipedia.


Yes, Thunderbird is built exactly like Firefox – it uses Gecko to render both the "contents" (email messages themselves) and the "chrome" (the whole Thunderbird UI).

(Don't forget that the original "Mozilla" browser, as well as its Netscape predecesor, had the web browser and the mail client in a single program, and Thunderbird is essentially just the mail component split out.)

Though you'll notice that the email preview windows are severely restricted (e.g. no JavaScript, hardly functional forms, all links forced to open in a new window), but that is deliberately done for security reasons because we just can't have nice things – the whole email message is still rendered using Gecko. (And if you use Thunderbird as an Atom/RSS feed reader, it will show the linked webpage.)

There were indeed plans to use pdf.js inside Thunderbird, and apparently there is a "Conversations" addon which can also provide this feature.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .