The pref you are referring to seems to be commented out, https://dxr.mozilla.org/mozilla-release/source/modules/libpref/init/all.js#61 . So you can just add it in about:config manually. See the comment surrounding the pref definition for possible values.

And how can I add it manually?

See also:

• https://dxr.mozilla.org/mozilla-central/source/netwerk/cache2/CacheObserver.cpp
• http://kb.mozillazine.org/browser.cache.disk.parent_directory

Type about:config in the location (address) bar and press the Enter key to open the about:config page like you open a website by typing the URL in the location bar.

• http://kb.mozillazine.org/about:config
• if you see a warning message then you can confirm that you want to access the about:config page

• use the Search bar at the top of the about:config page to locate preferences more easily
• preferences that have been modified and do not have the default value show as bold (user set)
• preferences that are user set (bold) can be reset to the default value via the right-click context menu
• preferences can be modified via Modify (String or Integer) or Toggle (Boolean) in the right-click context menu and by double-clicking the line with the pref
• preferences can be created via the right-click context menu: New -> String, Integer, Boolean

Thank you for your answers, but I still do not understand how to do that.

Explain what you don't understand. cor-el's answer covered everything.

Dear jagan605, You told me that I can manually add browser.cache.memory.capacity in about:config and gave me the link (https://dxr.mozilla.org/mozilla-release/source/modules/libpref/init/all.js#61). Unfortunately, I do not know how to youse it and how to add the code to the FF. Could you explain it to me I will be very grateful to you! Thank you!

Since you have missed cor-el's response, I will link it here. https://support.mozilla.org/en-US/questions/1185678#answer-1030302

Please read that answer. You just have to create the pref browser.cache.memory.capacity and specify the value. You don't have to modify any code.

Thank you to everyone! I did not really think that the solution is so easy!

In about:cache?device=memory page, at least until exist in FF 52 ESR, the modified browser.cache.memory.capacity value does not get reflected.

Is there any other way to verify the change of memory cache size?

See also:

• https://dxr.mozilla.org/mozilla-central/source/netwerk/cache/nsCacheService.cpp#907