Suggest Download the Developers Edition. https://www.mozilla.org/en-US/firefox/developer/ At the end of the install it will ask if you want to have compatibility mode to run the other, say yes. Can run both at the same time. No idea about Sync, do not want to go there.

A nice new Blue Firefox Icon will help tell the difference. No need to run the tools unless need to. Tools in 57+ also but not as good as above and the Firebug in 52, might be better.

Please keep your Video Card Drivers Updated, very important. Thank You.

Please let us know if this solved your issue or if need further assistance.

fasterbetter said

I have Firefox 57.0.1 x64 and Firefox 52.5.0 ESR x86 installed on my computer. (52 for using a old NPAPI plugin) Every time I switch from 57 to 52 or the other way around, the settings, homepage, bookmarks, etc get reset to default. Any bookmarks that I add or remove are synced across both browsers automatically (even though i have not set up FF sync). Also, I can't seem to be able to use both browsers simultaneously - If any one is running and I try to open the other browser, it just opens a new tab in the currently running browser. How can I make both the browsers run completely separate?

This is due to sharing the same Profile with 52 ESR and 57.0.1.

Unless you have each Firefox start with a specific Profile the two versions will just use the last Profile used.

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

The easiest thing to do is to set your desktop shortcuts for each version to start with their own separate Profile. http://kb.mozillazine.org/Shortcut_to_a_specific_profile

I say avoid using the dev edition as suggested above unless you want to be a Tester. It is basically same as Beta channel but with dev edition features. It gets say nine to fourteen Beta builds of a version on the channel in updates on average.

James said

This is due to sharing the same Profile with 52 ESR and 57.0.1. Unless you have each Firefox start with a specific Profile the two versions will just use the last Profile used. https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data The easiest thing to do is to set your desktop shortcuts for each version to start with their own separate Profile. http://kb.mozillazine.org/Shortcut_to_a_specific_profile

I read that but it only shows how to add/ remove profile. What lines do I need to add to the shortcut's target's to actually have it launch their respective profiles? I searched around and was finally able to have both running simultaneously but then the profile manager always launched first when I launch 52 ESR. What a mess. Meanwhile, Chrome Stable, Beta, Dev, and Canary all run in parallel and also keep userdata separate :/ Mozilla pls. Now that quantum runs as good as chrome, I tried to move to it but there's always something that finally results in me reverting back to chrome.

The ProfileManager can only run if Firefox is not running. unless you use the -no-remote switch for second instance with a separate Profile but that just complicate things.

The http://kb.mozillazine.org/Shortcut_to_a_specific_profile article I linked to shows you how to do it with the shortcuts.

Otherwise if you still did not get it then you could use portable Firefox for one of the versions as it is self contained and can even run off a usb flash drive.

https://portableapps.com/apps/internet/firefox_portable https://portableapps.com/apps/internet/firefox-portable-esr

I actually did read that article but the browsers were still not running as separate instances. I tried the -no-remove switch and it seems to be working. Hopefully, the bookmarks and other stuff does not get mixed up. Thanks for your help. btw, are there any disadvantage to using the above switch?