View, add, modify HTML code in Thunderbird, even without addons
If you have basic knowledge of HTML you may like to “fine tune” some email messages when you compose them in Thunderbird. There exist a couple of Addons for that, but they don’t work always! Obviously their developers are not very interested in updating them following the development of Thunderbird.
Fortunately, you can view, add and modify HTML code partially or completely from Thunderbird’s own “Insert” > “HTML” menu. Insert HTML is more powerful than it seems. If you just open this menu and click on HTML a small empty window opens waiting for input. Whatever HTML code you write or paste there it will be inserted in your message at the point of the cursor. This isn’t its only use though.
If you select a part of your message before you click the Insert HTML menu, then this small window is filled with the HTML code of the selected part of your message, waiting for you to make modifications. And, if you select the whole message, you see there all the HTML source, to be modified or replaced.
The only problem with this HTML editing provided by Thunderbird is that HTML code is not highlighted, which means that if your message is complex and you need to make a lot of adjustments you will have perhaps some trouble locating what you try to change. Other than that you can edit the source just fine.