@Maverynthia - Development Version Rated 5 out of 5 stars
@Maverynthia - Please read my previous review describing Mr Tech's development version which will install in FF6.* to 8. Please also take the time to read that referenced and recommended section of the Mr Tech Toolkit Mozillazine forum topic. Also note that there are at least two other extensions which can override version checking. They are mentioned in the forum. When considering add-ons, it can often help to read their previous AMO reviews.
Development Version for FF5 etc Rated 5 out of 5 stars
@Patrick et al
In the Mozine topic for this extension, there is a link to a development version which is compatible with FF5 and beyond.
[Ext] MR Tech Toolkit 6.0.4 (11-9-09) • mozillaZine Forums (Development version)
A lot of it is functional but the core functions such as specialised add-on display together with 'File'->'Install.., 'theme switcher', 'make compatible', per extension compatibility overrides, install date/time and similar are not yet working. Read all that Mozine topic from that post onward before attempting to try it for yourself. You will see what still functions, that Firefox provides alternatives to some of the missing features and some alternative extensions are suggested for individual missing functions. I find that the development version still provides many features I want and I only needed to install two other extensions for particular missing features. Otherwise, Firefox itself and 'All-in-One Sidebar', which I already used, fill the other, hopefully temporary, gaps. As for Mr Tech appearing to abandon, I don't think that has actually happened. The new Firefox Add-ons Manager is very different from the original and must have made it extremely difficult for him to adapt MTTK to fit it. On top of that, now that Thunderbird has more recently adopted the new manager, he has more than double work. I believe that he is still trying and we can hope to see a new, fully functional MTTK in due course. Meanwhile, read that information on Mozine and see what actions suit you best.