hi Ken-T, please get in contact with the addon's vendor to inquire how they'll make available a verified version of their addon in firefox (which is required in current versions of firefox and has been communicated to developers for months). thank you!

If your Delicious Bookmarks 2.3.4 extension is disabled / inactive due to that extension not being "verified" / not being "signed" ... see this KB article.

https://support.mozilla.org/en-US/kb/add-on-signing-in-firefox#w_what-can-i-do-if-firefox-disables-an-installed-unsigned-add-on

Override add-on signing (advanced users): You can override this setting by changing the xpinstall.signatures.required preference to false in the Firefox Configuration Editor (about:config typed in the URL bar). Support is not available for any changes made with the Configuration Editor so please do this at your own risk.

Note: That pref will be gone in Firefox 44 (Release slated for Jan 26. 2016) so user's of currently un-signed extensions should contact the developers of those extensions and prod them into getting their act together & get their extensions signed by Mozilla. Otherwise the users of those extensions will be SOL come the release of Firefox 44. Mozilla has already pushed back the version in which this new security feature was to be enabled from Firefox 40 to 44. I doubt if there will be any further reprieve.