I currently use Thunderbird and it doesn't have the capability to show my email threads as I'd like. I want to see ALL threads, including any emails I sent, in one view. Right now, I can see threads by folder, but it doesn't bring in any sent emails in my sent items folder. Is there an email client out there that can show me all threads in a single view?

hum; the purpose of folders is to segregate. Threaded views collects like topics in the current folder.

You can reorg the mailbox into one fold and use threaded views, but highly doubt
a client reader will thread across folders.

That is exactly the main reason why I DON'T like Gmail from Google, but it does exactly what YOU want (the threads).
Except you can't sort the damned thing!