It's in about:config.

Pref's name: browser.cache.disk.capacity

See also the about:cache page for detail about the current cache usage.

somaji said

In the recent versions of FireFox, I am unable to find cache settings. I, typically, set cache to 50MB. Where is the setting in the setting in Firefox Quantum (v61.0.1)?

Those settings used to be in the 3-bar menu (or 'Tools') - you now can modify cache-related preferences on the about:config page, as mentioned by TyDraniu.

In case you've never visited that page before, here's how :

Type in the address bar about:config and press Enter (promise to be careful, if asked)

Type in the search bar : browser.cache.disk

Note : The maximum browser.cache.disk.capacity = 1048576 (1024 MB)

After changing a pref's value, always close and restart Firefox.