"Component is not available" nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)"

I found some compatibility issues in Firefox browser.

Environment
Browser: Firefox 60.7.0esr (64 bit)
Browser download link: ftp.mozilla.org/pub/firefox/releases/60.7.0esr/

I try use these codes to fix this issues:

var _getComputedStyle = getComputedStyle(target, '::-webkit-scrollbar');
var width = '0px';
var height = '0px';
try {
    width = _getComputedStyle.width,
    height = _getComputedStyle.height;
}catch(e) {
    width = '0px';
    height = '0px';
}

Can these codes be merged into the master branch?

Start by reproducing the reported compatibility issue in Firefox 60.7.0esr using the code shown in the issue, then locate the relevant utility entry point and existing browser tests. Done means the Firefox error is addressed without regressions and the proposed change has a clear maintainer-approved path into the master branch.