I was having trouble switching and logging out of Twitter so I thought I could just clear site cookies to make it work, but I weirdly discovered "about:preferences -> Manage Data -> Remove Selected / Remove All Shown" has no effect. i.e. after clearing, when I open up manage data again, they are still there. I did not try "Clear Data" above it because I was afraid of clearing out all my logins.

I reinstalled my computer several weeks ago and current profile was directly copied from my last machine, so this might be a possible cause? Could someone be so kind to help me get remove cookies and hopefully twitter back to work? Thank you in advance.