What are ways to prevent Firefox saving data to the C:\ drive and using alternative drive or some exact directory only?
How to make Firefox portable?

(I tried special FirefoxPortable edition from PortableApps.com, but that firefox was lagging when profile got bigger and many open tabs.)

Here i describe how i moved my profile folder to different location

I see on Windows 10 when i do Win+R and execute: %AppData%\Mozilla\Firefox
It shows Firefox is using following path to store its profile data: C:\Users\myname\AppData\Roaming\Mozilla\Firefox

There is profiles.ini file which i can tweak (when Firefox closed) to contain example:

And then move my profile folder contents to this new location. Example contents: bookmarkbackups, browser-extension-data, cache2

I assume FF will continue to rely on %AppData%\Mozilla\Firefox but there it will be redirected to different location.

But is this enough to prevent Firefox saving any data into different folder than its own folder or profile folder?