Tested similar add-ons and found out this is the only one that actually works. Copy & Paste works although you can't use CTRL-V for pasting images. Use right-click's paste function instead.

Thanks Wolvan.

Edit:
Unfortunately video's do not show up.
Also try add-on Twitter Old UI

It does, in fact, bring the Old Twitter Layout back. It claims that Paste function has been restored, but it hasn't, hence 3 stars.

This add-on (and a similar) appear to work well, but have one big problem. They prevent embedded videos in some Twitter posts from being viewable. If the add-on is disabled, they work. The symptoms are: (1) the static image for the video flashes very briefly on the screen, but (2) then I get a gray player box saying "We cannot play the video in the browser. Try another web browser."

Here is an example: https://twitter.com/GreenRupertRead/status/1182979393666375680.

In Web Developer | Web Console, I get this further explanation:
<>
Cannot play media. No decoders for requested formats: video/mp4; codecs="avc1.42E01E mp4a.40.2", video/quicktime; codecs="avc1.42E01E mp4a.40.2", video/x-m4v; codecs="avc1.42E01E mp4a.40.2", application/x-mpegURL, video/mp4; codecs="avc1.42E01E mp4a.40.2"
<>

I hope you can find a way to fix this. (It's the only reason I didn't give it 5 stars.)

As of 8/13 the paste function is broken in most text entry fields. If I'm on the home feed and click the "Tweet" button in the upper right, pasting works, but it doesn't work in any other context. I can't paste anything (not even text!) in replies, I can't paste anything from any other feed than the top right button on the home feed. The reply/retweet buttons on the home feed are becoming unresponsive, too. I'll have to click them multiple times (or refresh the page entirely) to get them to work.

copy-paste not work 60.6.1esr (64bits)

I am writing a new tweet,
Before writing a tweet, I copied the text into the clipboard.
I want to paste the text from the clipboard into the tweet text.
Text insertion does not work
FireFox Portable 60.6.1esr (64bits)