Is there an add-on/extension/plugin that converts a plain text link into a clickable hyperlink?

"Linkification" is one possibility. The publicly released version isn't up to date but the reviews point to the website with an up to date version. https://addons.mozilla.org/en-us/firefox/addon/linkification/ I have used it in the past without problems.

*Text Link: http://piro.sakura.ne.jp/xul/_textlink.html.en *CoLT: https://addons.mozilla.org/firefox/addon/colt/ *Copy Link Text: https://addons.mozilla.org/firefox/addon/copy-link-text-4750/ *Go to Selection: https://addons.mozilla.org/firefox/addon/go-to-selection/ *Multi Links: https://addons.mozilla.org/firefox/addon/multi-links/ *Selection Links: https://addons.mozilla.org/firefox/addon/selection-links/

Thank you very much. That one didn't work with my version of Firefox, but it did lead me here (http://yellow5.us/firefox/linkification/) which works perfectly.