Birthe,
Thanks for your quick answer. I tried Firefox 57 and Firefox ESR on Windows and an old version with Tiddlyfox on Lubuntu (linux) with no success. A few minutes ago I tried the following (also with no success):
1) Saved
http://tablinks-ff.tiddlyspot.com/ (the plugin version for Firefox) to my computer. Clicked on the tab On/Off, and on the toggle button, I click on and off, but in both cases nothing happens (the plugin is active and I can't disable it).
2) I did the same with
http://tablinks.tiddlyspot.com/ (the plugin version for Chrome) and it worked (I can disable the plugin).
I wonder what am I doing wrong.
I know there is also Mario's
https://wikilabs.github.io/editions/link-to-tabs/ but it's not exactly the same.