[Feature] Disable firefox auto translate popup by default

[atomfrede]

With the latest firefox supports auto translations. By default the auto translate popup will be shown (e.g. my os is in english, but I test websites in german). It can happen, that the auto translate popup is shown above elements you need to click. In that case the test fails.
In particular for me it looked like a regression, because tests worked fine until the recent update.

One can set it manually via

 launchOptions: {
      firefoxUserPrefs: {
        "browser.translations.automaticallyPopup": false,
        "browser.translations.enable": false, 
      }
    }

but I think it would be a good default.

[atomfrede]

@mxschmitt If you like I can do a PR too (it looks like a pretty small change in /playwright.cfg if I am not mistaken).