Doing anything in Firefox has nothing to do with Thunderbird. They are two separate, independent programs. Reinstalling Thunderbird rarely fixes anything since your data and settings are in different locations. A new install just picks up the existing settings file and you are right back where you started.

Are you just missing a toolbar in the Composition window? Click the Write button to open a Composition window. If the Menu Bar with File-Edit-View etc is not showing press the alt key to make it appear. From the Menu Bar select View-Toolbars and turn the missing toolbar back on. The Send button is on the Composition toolbar.