Hello 4rs4,

Certain Firefox problems can be solved by performing a Clean reinstall. This means you remove your Firefox program files and then reinstall Firefox. This process does not remove your Firefox profile data (such as bookmarks and passwords), since that information is stored in a different location.

To do a clean reinstall of Firefox, please follow these steps: Note: You might want to print these steps or view them in another browser.

1. Download the latest Desktop version of Firefox from mozilla.org (or choose the download for your operating system and language from this page) and save the setup file to your computer.
2. After the download finishes, close all Firefox windows (or open the Firefox menu and click the close button ).
3. Delete the Firefox installation folder, which is located in one of these locations, by default:
   • Windows:
     • C:\Program Files\Mozilla Firefox
     • C:\Program Files (x86)\Mozilla Firefox
   • Mac: Delete Firefox from the Applications folder.
   • Linux: If you installed Firefox with the distro-based package manager, you should use the same way to uninstall it - see Install Firefox on Linux. If you downloaded and installed the binary package from the Firefox download page, simply remove the folder firefox in your home directory.
4. Now, go ahead and reinstall Firefox:
   1. Double-click the downloaded installation file and go through the steps of the installation wizard.
   2. Once the wizard is finished, choose to directly open Firefox after clicking the Finish button.

More information about reinstalling Firefox can be found here.

WARNING: Do not use a third party uninstaller as part of this process. Doing so could permanently delete your Firefox profile data, including but not limited to, extensions, cache, cookies, bookmarks, personal settings and saved passwords. These cannot be easily recovered unless they have been backed up to an external device! See Back up and restore information in Firefox profiles.

Please report back to say if this helped you!

I did that and it did not fix it. A fresh install of 62.0 (64-bit) is still broken. Copy/paste only works inside Firefox, things copied outside dont show up when pasting inside it, and vice versa.

Looks like this might have been related to my desktop security setup. I am logged in via a network profile associated with my employer; we recently had to change our user passwords for work, which is also the user login password on our desktop & my Mac. It seems that despite my user login password being updated, the "accountsd" password was not updated, and was still using the old password. Eventually on boot a pop-up box appeared asking to enter the password for "accountsd"; I put in the old password, and proceeded to update the password on my "login" Keychain to the new one. Now the copy/paste seems to be working again. I guess this is a security restriction that macOS has in place or something.