Threads is a toggle that can be switched off using the toggle icon

Ahh… I was wondering why I was seeing so many questions lately asking how to remove threading, because I'm used to change that setting using the View options, but now I'm starting to think what's happening is that many users click that icon, not knowing what it does, and see the effect later when they no longer associate the threading with having clicked that icon…

Thanks for those replies, but there's a different layout for my Linux version.

On the inbox heading area I don't have a second line for Subject and Correspondents.

The only icons are Quick Filter and Sort. There's none for Threads.

In View > Threads, there is a check for All which I cannot remove. Same situation with Sort > Sort by > Threaded.

So I'm still stuck.

A list of Thunderbird's Keyboard shortcuts can be found by searching in the top right search window on this page.

They're located at https://support.mozilla.org/en-US/kb/keyboard-shortcuts-thunderbird.

See if they work with Linux - there are numerous ones to deal with threads.

there's a different layout for my Linux version

That's because of different view options, not because of different OS.

On the inbox heading area I don't have a second line for Subject and Correspondents.

You're probably using Card View instead of Table View:

https://support.mozilla.org/kb/getting-started-thunderbird-main-window-supernova

In View > Threads, there is a check for All which I cannot remove. Same situation with Sort > Sort by > Threaded.

It's in View > Sort By. Can't you choose Unthreaded instead of Threaded there? That option should be available in both Card and Table views…

The following article describes threading in Thunderbird:

https://support.mozilla.org/kb/message-threading-thunderbird