There is a Finnish version of this article named ‘Kaksin aina kaunehimpi – vanha ja uusi Firefox rinnan’
How to install old and new Firefox (i.e. XUL/XPCOM and Quantum versions) side-by-side
The following instructions are for advanced power users who really feel they need both versions. Users of the old add-ons have reached a point where they need to move on: some newer pages no longer work on the old browser, or the security level of the old browser is not sufficient to handle financial matters, etc. There may still be reasons to continue using the old browser because of the very useful add-ons. The idea is to install both versions on the same machine so that they work well separately without interfering with each other and, if necessary, complement the other version with a more appropriate feature.
Following these instructions carefully will ensure that the system remains in good working order. If you do not understand the instructions, do not change anything until you are sure of what you are doing.
IMPORTANT: Make sure you back up properly before making any changes: for example, zip the Firefox profile folder and make copies of the files you are going to change so you can easily revert to the previous situation.