Hold Shift when you launch Thunderbird, and when the Safe mode window appears, check Reset toolbars and controls and click Make changes and restart.

Does that make the Forward button active?

ref: https://support.mozilla.org/en-US/questions/1169412

Dear Fabián: Thanks for your kind reply, but no... it didn't work although I did exactly as you suggested ;)

re :the "forward" button is greyed.

Try this:

• select email so it opens in Message Pane.
• Right click in header area and select 'Customise'

Buttons will become highlighted A new 'customise toolbar' window opens

• click on 'Restore Default Set'
• Click on 'Done'

see image below as guide.

Hi Toad, THANKS! Your reply did solve the problem. In fact, there are two "forward" buttons (dunno why) and I had the wrong one on the toolbar! Thanks again and greets!