For the most part, add-ons (extensions and themes) are trouble free. However, once in a while an add-on will not install, upgrade or uninstall correctly. Or may be you launched a different version of Firefox on the same profile and now your add-ons are disabled because Firefox says they are incompatible.I will cover the three most common issues with Add-ons Installation: Software Installation Disabled Extension(s) not installed after restarting Firefox Add-on(s) Disabled by Firefox Software Installation Disabled This issue mainly occurs when upgrading to a major version of Firefox such as going from 0.* to 1.0.* or 1.0.* to 1.5.0.*…