That does not appear to be a Thunderbird issue, but a PDF viewer issue.

I agree it is a PDF viewer problem but as I said, I opened the PDF in Thunderbird with the Thunderbird/Firefox PDF viewer. Firefox is the default app for PDFs. The other tools in the drop down menu - rotate clockwise, anticlockwise, hand tool etc. all function. When clicking on the "Text Selection Tool" the menu blanks but then the pointer will not select text. Selecting the hand tool, the hand symbol appears as the pointer and can be used to move the document on the screen. The Text Selection Tool is not working. I have opened other PDFs and the Text Selection Tool does not function on any of them.

Have done some experiments. Set default PDF viewer to either Adobe or Firefox. Contrary to previous post, text select tool works with either viewer for PDFs from local file or from downloads using Firefox. However select tool does not work on opened PDF attachment to an email in Thunderbird with either viewer. Problem appears to be a missing link between Thunderbird tools menu and the PDF viewer for the text select tool.