First, go to tools>preferences>general, scroll to bottom of right pane to 'config editor'. Open that and locate network.protocol-handler.warn-external.https and network.protocol-handler.warn-external.http entries. Change from false to true for both. Next, go to tools>preferences>general and locate the files and attachments section. Locate the http/s entry, right-click and specify your browser. (Alternatively, if entry for http/s is not there, locate an email with an html reference and open with right-click, where you can specify the default.)

The problem,as I indicated, is that Preferences are not available in the beta version. My older but perfectly functioning (since 2007) version of Thunderbird became corrupted with the Nov. 1, 2021 update, and I ultimately had to reload from scratch. The current version does not have Preferences or Options anymore. I cannot understand why the developers removed Preferences! I used it frequently both to request Return Receipts and to send an e-mail copy to myself. i have loved Thunderbird, but I am considering changing to another e-mail client. I also lost a large portion of my address book when I imported my data into the beta version. I am very frustrated!

In the beta version, the word 'preferences' was changed to 'settings' - so the menu would be for tools>settings and not tools>preferences.