I'm just testing, if your App RecentX is interesting for me - and the attaching of files to emails works great with gmail, but not within Thunderbird. Because I do all my email (including gmail) in Thunderbird. Am i doing something wrong or does RecentX not work with Thunderbird?
RecentX works with Thunderbird as well. You will be able to attach files to Thunderbird compose window using RecentX drag and drop.
Drop the file in the upper area (gray box with From, To, etc.).
You can also check out this short video for more clarity: