I have a heavily customised FF v42 setup I am happy with but my Firefox version is seriously out of date and it's time to upgrade. I suspect a lot of things will no longer work on the latest version (particularly some of my old addons). To ease the transition I want to be able to run my old version (temporarily for say a few weeks) and the latest one simultaneously so I can customise the new one the same standard and find solutions for anything that is no longer supported and still use my old install/addons until that transition is completed. I found this thread: https://support.mozilla.org/en-US/questions/1061740 but the solution pointed to has links that no longer work. I couldn't find anything recent on the web or youtube that covers the Mac. I understand the principles will be to: 1. Install current version in a different location. Is that just dragging the FF .app file to somewhere other than the Applications folder or is it asked as part of the install? 2. Use a different profile for each install. Not sure the sequence of doing this so that I don't lose my current (old) profile. I'm guessing it's create a new profile, switch to the new profile and then install current version which will "take over" the new profile. Then I can switch back to the old one when I run my my old FF? Any help greatly appreciated.