For changing download location, see my reply to this question: [https://support.mozilla.org/questions/1087883 Change download location]. I don't know about offline browsing. What do you mean that? You might search around on other Android sites for tweaks if you don't find what you're looking for here.

Any more tweaks can be done in about:config ???

Probably dozens of things, but my mind doesn't work like that.

Please tell some changes which you have done in about:config

I can't recall what changes I've made for myself, but here are some past answers involving about:config on Android. No guarantee that these are useful to anyone or still work in the current version! * Block automatic image downloading: https://support.mozilla.org/questions/1030877#answer-652949 * Lie to sites about whether you have a phone or tablet: ** Gmail: https://support.mozilla.org/questions/1053476#answer-707866 ** YouTube: https://support.mozilla.org/questions/1058016#answer-719568 (see the poster's follow-up) * Turn off form and search history (same setting different situations): **Disable the form history / form fill feature: https://support.mozilla.org/questions/1012939#answer-609498 ** Prevent saving of search history: https://support.mozilla.org/questions/1004055#answer-584750 * Turn off the popup blocker: https://support.mozilla.org/questions/1024438#answer-638462 * Change link colors (on pages that don't override the default setting): https://support.mozilla.org/questions/1064007#answer-733313 * Use a standard list of fonts instead of page fonts: https://support.mozilla.org/questions/1056938#answer-725384 * Turn off URL fixup: https://support.mozilla.org/questions/1052495#answer-704904 * Wrap long lines if you use view-source in a tab: https://support.mozilla.org/questions/1089484#answer-795623 * Disable malware and phishing protection: https://support.mozilla.org/questions/1090533#answer-798195