Not sure exactly what the problem is, as 70b1 works fine here. Run in safe mode (hold Shift when you launch TB) to check if there's a broken layout.

re:In the future, please check TB's integrity and operation before uploading to the public.

You have chosen to download and run using a beta version which is not the current release.

The purpose of people using beta versions is to test the product, find and report defects/bugs so they can be fixed prior to actual release. This is the latter stages of testing software and requires a greater number of users to participate than can be tested by a handful of developers. Your participation in this process is much appreciated and bugs can be reported in Bugzilla. Please provide OS, version of Thunderbird, what you did, what occured and what was expected. https://bugzilla.mozilla.org/home

Most people understand that beta versions are not fully tested and they are put out to the public so they can help locate any bugs. So expecting a beta version to be fully tested and operational is never going to happen.

If you want to use the current release - version 68 - then download from the official webpage: https://www.thunderbird.net/en-US/thunderbird/all/

Has anyone noticed the XUL error that is shown on the picture?

  < toolbarbutton

-- ^

(Sorry, can't pasted it one-to-one)

Where does that come from? Some faulty add-on?