Hmm, normally double-click selects a word and triple-click selects a paragraph -- there's a setting to switch triple-click to only selecting the current line; only selecting the current word isn't one of the options.

I wonder whether the first click is getting absorbed/lost for some reason. Could you try Ctrl+A (select all) to see whether the cursor is in the page before you double-click?

Yes, the cursor is in the page. I have tried selecting several times.

I will now try uninstalling and reinstalling Firefox.

If this problem just started, you might shut down and restart Windows first. Sometimes there's a device glitch that lasts until you restart the system.

This problem has been for some time and the laptop has been restarted many times since the problem started.

Uninstalling and reinstalling didn't help.