Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem (switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance).

• Do NOT click the Reset button on the Safe mode start window or otherwise make changes.

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

Thank you kindly for your assistance. Unfortunately, restarting Firefox in Safe Mode did not solve the issue. Please let me know what else I can do. Thanks bunches!

Does this also happen if you try to copy/cut and paste text in a text area like "Post a Reply" on this forum?

Try to reinstall Firefox.
Download a new copy of the Firefox program and save the disk image (dmg) file to the desktop

• Firefox 19.0.2: http://www.mozilla.org/en-US/firefox/all.html

• Trash the current Firefox application (e.g. open the Applications folder in the Finder and drag the Firefox application to the Trash) to do a clean (re-)install
• Install the new version that you have downloaded
• https://support.mozilla.org/kb/Installing+Firefox+on+Mac

Your profile data is stored elsewhere in the Firefox Profile Folder, so you won't lose your bookmarks and other personal data if you uninstall and (re)install Firefox.

• http://kb.mozillazine.org/Profile_folder_-_Firefox

www.mozilla.org/en-US/

Happily, I was able to copy and paste the above Mozilla web address into this box. What I cannot do is copy and paste into Twitter reply boxes. I wrote Twitter about this. They sent me to you instead.

Do I still need to reinstall Firefox?

Thanks!

Try to use the keyboard shortcuts if the buttons on the web page or other methods aren't working.

• Copy: Command + C
• Paste: Command + V
• Cut: Command + X

• https://support.mozilla.org/kb/Keyboard+shortcuts

Hmm. Sadly, it didn't work. I've gone back to the Twitter folks and asked them about this again. I will let you know what they say. I really appreciate your help. Thank you again!

Is that a plain text area like on this forum or a rich text area that also accepts HTML code?

• italic text example

You can't do anything fancy on Twitter, so I guess it's plain?

Yes, that sounds reasonable.

Does it have any effect if you temporarily disable JavaScript after the page has loaded?

• Firefox > Preferences > Content : Enable JavaScript

Yes! I can copy and paste, but it remains greyed-out, and won't post. Might be onto something here... I hope! What now?

That would be an indication that some JavaScript that is running on that page is blocking or intercepting those keys.
You would have to ask Twitter to look into this.

It took some time, but Twitter did reply and said they are aware of this problem and they are working to repair it.

I really appreciate your help.

Thank you!