Sorry,but i think firefox can't put tabs on bottom

Old versions supported this option. There was "Tab on Bottom" add-on

Hi

That add-on has not been supported for quite a few years.

You need to use a file userChrome.css. There is code on private sites like github or you could try the forum version. https://support.mozilla.org/en-US/kb/forum-response-tabs-below-url-bar

WARNING: This script is not provided by Mozilla and is not officially supported Creating a userChrome.css file and other ways of customizing Firefox that are not exposed in the interface are there for developers, not end-users. If it were intended for end-users, you would see a menu option or checkbox; you wouldn't have to hack a text file in a hidden location.

What this means is that even though the functionality you want is no longer supported, a third-party has found a way to do it using the CSS file. If it breaks in new versions, you will need to contact the person who provided the script.

[Warning added by moderator.]