Delete the files extensions.* (extensions.rdf, extensions.cache, extensions.ini, extensions.sqlite) and compatibility.ini in the Firefox profile folder to reset the extensions registry. New files will be created when required.
See "Corrupt extension files": http://kb.mozillazine.org/Unable_to_install_themes_or_extensions

If you see disabled, not compatible, extensions in "Tools > Add-ons > Extensions" then click the "Find Updates" button (in Firefox 4: right-click the extension -> "Find Updates") to do a compatibility check.

You may also need "Reset all user preferences to Firefox defaults" on the Safe mode start window.

Thanx for quick response. Tried all that. No change. All Extensions, Themes, AND PLUG-INs (I just noticed the inclusion of the latter) are still "disabled for my protection". Also, keep in mind I'm stuck in FF 3.5, not 4.x.

Wait, amend that. Extension and one theme (out of five) updated. Plugins, however, present no option to update in Add-On window. Have to restart broweser now, will update.

Updates change nothing, after brief moment of hope.

Is it my entrie version of FF (3.5.6) that is regarded as too out of date, and therefore insecure?

My installation situation is that I'm running an older version of MEPIS, a Debian-based Linux; Debian, as you may well know, typically prefers their own variant, Iceweasel, due to licensing and update methodology quibbles. I'm nervous about using Iceweasel because the version numbers are different, and I have no idea how they match up to FF versions. My specific distro does provide FF as FF, but they max out with the version I have currently, and do some system-level customizations (as I've found doing manual java updates). I'm reluctant to do a wholesale copy-drop of the Mozilla-provided tarball of FF3.6/4.x because it might overwrite some MEPIS-specific configurations like that; I don't want to lose, for example, the links (via /etc/alternatives) that point to my plugin directories (like my manual java updates)