Rather of installing an old version of Firebug, you should help us to track the problem down, so it can be fixed.
If you still want to use Firebug 1.12, there's an extension
Disable Add-on Compatibility Checks available, which allows you to do so.
Though note that there's no support for Firebug 1.12 anymore. So parts of it may not work as expected anymore (especially the
Script panel doesn't work at all anymore due to
old Firefox Debugger APIs called JSD being removed in Firefox 33.0) and it may break completely at any time. So it's not recommended to use it.
Sebastian