You can have as many Firefox programs on your computer as you wish. However, each program must have its own user profile.

Some data can be shared between each user profile using sync. However, some things cannot be shared, For example; Ad-ons and preferences.

Thanks for the reply, please note I wrote that I did create and switch to a new 'temporary' profile before switching to the older version of firefox, yet my 'real' profile was still corrupted.

To describe again: I closed 'new' firefox which was visibly switched to my new 'temporary' profile as the default profile, then opened 'old' firefox and I could see sqlite altering and corrupting the files in my 'real' profile folder immediatley as 'old' firefox (51) was opened.

Each Firefox program must be attached to its own profile. You cannot have one profile access by another program it will cause corruption

Sounds that you used both Firefox versions with the same profile.

Did you start Firefox 51 with the Profile Manager and create a new profile ?

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

On Mac there is more involved with using multiple Firefox versions and multiple profiles and you probably need to start Firefox via a terminal window. Firefox 67 and later use a dedicated profile and will refuse to use a profile used by a newer Firefox version. Firefox 51 doesn't complain and running Firefox 56 and older with a profile from a current release will corrupt a lot of files. You only noticed places.sqlite and favicons.sqlite, but there are more files not compatible and best is to create a new profile for the current Firefox release.

There should be no need to use Firefox 51 for Flash as Flash is still supported in current releases, you would only have to create a new profile if you need a safe or independent environment. If you really want to use an older version then look at Firefox 52.9.0 ESR or 60.9.0 ESR with their own profile.

• https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

Thanks both! Yes I did initially start old and new Firefox naively by double clicking the firefox executables. I didn't believe they would share the same profile by default since profiles live, profusely, in named subfolders.

After realising both firefoxes did share the same profile, I tried to change the profile that was enabled in new firefox before opening old firefox (surely understandably, if you're following my logic: there seemed to be a configuration pointing both/any firefoxes to one specific profile).

But this too corrupted the profile. (So why does old firefox choose to activate my 'real' profile in this situation, what determines that it choses that one of numerous profiles in my profile folder? It doesn't appear to be alphanumeric?)

But yes, understood, the best way is to ignore 'new' firefox entirely, and start old firefox in profile manager mode with `-P`, select a new/different Profile, which 'old' firefox will then always use, allowing 'new' firefox to continue to use my 'real' profile. So where is the link between firefox executable and profile-to-use, stored?

(sorry I mentioned Flash, that was confusing, in fact I need to use Java Applets, this is why i'm using Firefox 51)

You are correct about creating a new profile,

As to what Firefox version you want to run, do this;

Each Firefox must have its own batch file program to send the necessary information into the browser when started.

Here is an example (on one line only); Windows: C:\Program Files (x86)\Mozilla Firefox Current\firefox.exe -no-remote -P Firefox_v_57

path to Firefox version \ command switches

The 'new' Firefox version (71.0) doesn't have problem with using the profile from the older Firefox version and will happily upgrade this profile and that makes the profile incompatible with the older version. Both versions may use the profile marked as Default=1 in the profiles.ini file although Firefox 71 would use its preferred dedicated profile is such a profile is set. What might work is to first start Firefox 71 via the Profile Manager and select its profile to be used. This should store this choice in installs.ini as the preferred profile for this version. Start start Firefox 51 via the Profile Manager and make sure its profile is selected and that this profile is used by default. If you now start Firefox 71.0 then it should use its dedicated profile as found in installs.ini. Firefox 51 will use the profile set as default in the Profile Manager as it doesn't know about dedicated profiles and that should be the profile you selected previously. Firefox 71 can scan all available profiles to see what Firefox version last used a profile and select the profile that had previously been used by this version.