Click Format>test style>strikethrough from menu bar

I don't know about an icon, but it's possible to create a key shortcut with the tbkeys-lite add-on. The attached picture shows how to add the code to the tbkeys settings. In this example, Alt+Z makes a strikethrough in selected text in the compose window.

Hi, @sfhowes: also a solution, thanks, @david: as I mentioned in my post, my opinion is that it is too complicated to go through the Menu. I feel, as there is a icon for bold, italic and underline, it would be nice to have also a icon to strikethrough.

I tried but could not get the add on to work. Yes it would be nice to add that to the tool bar.

In the good old days there was an extension that did this, along with icons for sub- and superscript. Sadly it was killed off by the changes made to extensions a few years ago.

The best place to make suggestions is Mozilla Connect, though if any notice will be taken is a moot point.