The unread count for all folders can be hidden with css:

https://support.mozilla.org/en-US/questions/1426787#answer-1609463

For specific folders, try adding something like this to the code:

[data-folder-type="Trash"]

Something like this? [data-folder-type="Trash"] .unread > .container > .unread-count {

   display: none !important;

}

I'm not good at css, and this isn't work.

I used this css to check or I had configured all in the right way:

#folderTree {
   background-color: red !important;

} and I'm pretty sure, that I had configured all in the right way, but first css doesn't work

Try something like this:

ul#folderTree li[data-folder-type="trash"].unread > .container > .unread-count {
  display:none !important;
}

Reference: https://forums.mozillazine.org/viewtopic.php?t=3129527

It works! But, if I need use the same for some another folders, some certain folders that I need, how can I check its folder type?

Alsa I have a problem, that counter is invisible now, but name of the Trash folder is hightlight

To make the Trash folder not change:

ul#folderTree li[data-folder-type="trash"].unread > .container > .name {
color: black !important;
font-weight: normal !important;
}

trash can be replaced by other generic folders such as inbox, sent, junk, drafts. Not sure how to apply it to arbitrary user-defined folders. For those, consider unchecking 'When getting new messages...' in folder Properties.