Commerce Kickstart: facets are disabled when using back button
This is an issue with how FireFox serves cached pages when you use the Back button. 

There appears to be two ways of solving this:

1) Per the first answer here, set autocomplete="off" for all form elements that can be disabled. As that form likely isn't one you have direct control over (without hacking a module), you'll have to do this with a hook_form_alter(). 

2) The second answer in the question linked above provides some JavaScript that you can use to stop this from happening:

window.addEventListener('pageshow', PageShowHandler, false);
window.addEventListener('unload', UnloadHandler, false);

function PageShowHandler() {
    window.addEventListener('unload', UnloadHandler, false);
}

function UnloadHandler() {
    //enable button here
    window.removeEventListener('unload', UnloadHandler, false);
}