This one is modified from another addon.
I modified it a little, making it function on not only http web but other schemes also (eg. local files).

Original README:

Get The Best Browsing Experience Without Limitations And Restrictions In An Online World

★ Features :

→ Remove Copy Text Protection On All Website
→ Force Enable Right Click Button
→ Allow Copy And Highlight
→ Disable Annoying Dialog Message (Not Able To Copy Content On This Webpage)
→ Re-Enable Context Menu
→ Include "Absolute Mode" To Force Remove Any Type Of Protection
Made Possible By Absolute
Original link: https://addons.mozilla.org/firefox/addon/absolute-enable-right-click/

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Update:
I uploaded another one with simpler structure (no popup) and 1 more feature (editing web).
---->> Enable Select Copy +Edit

Install

• License: BSD 2-Clause "Simplified" License
• Weekly downloads: 131
• Average daily users: 1409
• Rating: 4.4286/5 of 7 ratings
• Created: 2021-04-15 05:58:29
• Last updated: 2021-05-15 04:00:26
• Homepage: https://garywill.github.io
• Support site and email
• Orig: https://addons.mozilla.org/en-US/firefox/addon/enable-copy-by-gary/
• API: {173ba6af-c12b-411d-98a7-84ba21d8062c}