There are multiple ways to forcing Firefox to load add-ons that were not designed for newer versions, one of the ways I found was the ability to turn of compatibility checking in Firefox altogether. I do not recommend this method, as automatic updates for Firefox may change something, and without proper compatibility checking, may render Firefox unusable. I personally haven't used as I prefer the alternate method addressed below, but for those who want to through caution to the wind, here is what you do to disable all compatibility checking:
- Open a new tab and type "about:config" (without quotes).
- After confirming that you won't muck about, in the Filter field at the top, enter "extensions.checkCompatibility" (without quotes). If there isn't an entry for it, right-click and select New > Boolean with the same name and set its value to "false". If it does exist, make sure its value is "false", using the right-click > Toggle item to change its state.
- Right-click in the list and select New > Boolean and give it the name "extensions.checkCompatibility.4.0" (without quotes) and set its value to "false" as well. If the value already exists, make sure you set its value to "false".
- Restart Firefox.
Please be aware that these do not guarantee that an older add-on will work with newer versions of Firefox. But I hope this helps someone else besides myself as I know its been a little painful being asked to update every time I start Firefox but not doing the upgrade because of seemingly incompatible plugins.