Erratic Taskbar

[Coppit]

With a few sites lined up I go to open another one (which exhibits), and the existing ones disappear. Sometimes they can be found, but more recently are closed.
With one site only open, trying to add a site by going into History and double clicking it, it arrives but replaces, not adds to, the existing one. What is wrong?

[SlackAlice]

//With one site only open, trying to add a site by going into History and double clicking it, it arrives but replaces, not adds to, the existing one. What is wrong? //
Nothing wrong, rather than double click, single right click and choose 'open in new tab'

[Fitzer]

Telling us which browser you're using may help people to provide you with an answer.

[Coppit]

Thanks SlackAlice. The disappearing tabs has not reappeared so far today.

[Coppit]

Firefox, Fitzer.

[Coppit]

The problem, which has been evident for a couple of weeks, has not reappeared so far. I should have waited one more day ... .