xul-ext-* are not compatible with Firefox 57+

[Norbert]

Steps to reproduce:
1. Install modern Ubuntu with all updates and all xul-ext- extensions
2. Launch Firefox 57 or newer
3. Go to Tools|Add-ons, Legacy Extensions and find Ubuntu Modifications 3.3 and others marked as LEGACY.

Expected results:
Ubuntu Modifications and others are compatible with modern Firefox.

Actual results:
Ubuntu Modifications and others are not compatible with modern Firefox, it does not fit Mozilla's trends (https://support.mozilla.org/en-US/kb/firefox-add-technology-modernizing?as=u&utm_source=inproduct).

Note:
for Ubuntu Modifications (ubufox) see bug 1711778.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: xul-ext-ubufox 3.3-0ubuntu2
ProcVersionSignature: Ubuntu 4.13.0-17.20-generic 4.13.8
Uname: Linux 4.13.0-17-generic x86_64
ApportVersion: 2.20.8-0ubuntu2
Architecture: amd64
CurrentDesktop: MATE
Date: Mon Dec 4 00:33:54 2017
PackageArchitecture: all
SourcePackage: ubufox
UpgradeStatus: No upgrade log present (probably fresh install)