The thing is, Gecko is really insecure when compared to Chromium. Its sandboxing is asinine. Additionally, due to lack of WebView implementation, on mobile you have to use Chromium either way, leaving you with two completely separate attack surfaces.
Quoting GrapheneOS developers[1]:
> Avoid Gecko-based browsers like Firefox as they're currently much more vulnerable to exploitation and inherently add a huge amount of attack surface. Gecko doesn't have a WebView implementation (GeckoView is not a WebView implementation), so it has to be used alongside the Chromium-based WebView rather than instead of Chromium, which means having the remote attack surface of two separate browser engines instead of only one. Firefox / Gecko also bypass or cripple a fair bit of the upstream and GrapheneOS hardening work for apps. Worst of all, Firefox does not have internal sandboxing on Android. This is despite the fact that Chromium semantic sandbox layer on Android is implemented via the OS isolatedProcess feature, which is a very easy to use boolean property for app service processes to provide strong isolation with only the ability to communicate with the app running them via the standard service API. Even in the desktop version, Firefox's sandbox is still substantially weaker (especially on Linux) and lacks full support for isolating sites from each other rather than only containing content as a whole. The sandbox has been gradually improving on the desktop but it isn't happening for their Android browser yet.
IronFox (an FF fork) developers[2]:
> While we do as much as possible to improve the situation, it should be noted that Firefox-based web browsers, including IronFox, have security deficiencies when compared to Chromium. This is especially notable on Android.
Quoting GrapheneOS developers[1]:
> Avoid Gecko-based browsers like Firefox as they're currently much more vulnerable to exploitation and inherently add a huge amount of attack surface. Gecko doesn't have a WebView implementation (GeckoView is not a WebView implementation), so it has to be used alongside the Chromium-based WebView rather than instead of Chromium, which means having the remote attack surface of two separate browser engines instead of only one. Firefox / Gecko also bypass or cripple a fair bit of the upstream and GrapheneOS hardening work for apps. Worst of all, Firefox does not have internal sandboxing on Android. This is despite the fact that Chromium semantic sandbox layer on Android is implemented via the OS isolatedProcess feature, which is a very easy to use boolean property for app service processes to provide strong isolation with only the ability to communicate with the app running them via the standard service API. Even in the desktop version, Firefox's sandbox is still substantially weaker (especially on Linux) and lacks full support for isolating sites from each other rather than only containing content as a whole. The sandbox has been gradually improving on the desktop but it isn't happening for their Android browser yet.
IronFox (an FF fork) developers[2]:
> While we do as much as possible to improve the situation, it should be noted that Firefox-based web browsers, including IronFox, have security deficiencies when compared to Chromium. This is especially notable on Android.
[1] https://grapheneos.org/usage#web-browsing
[2] https://gitlab.com/ironfox-oss/IronFox/-/blob/dev/docs/Limit...
An in-depth examination of this topic and a plethora of other sources can also be found here: https://madaidans-insecurities.github.io/firefox-chromium.ht...