Easier "runtime/on-demand" pref change support

[gregglind]

Is this a bug or feature request?

feature

What is the current behavior?

The easiest way to get prefs into the firefox startup is to clone a profile and modify prefs.js or friends.

What is the expected or desired behavior?

web-ext --prefs some.json|somePrefs.js --prefs someOther.json which would be multicallable and applicable in right to left order. Each of which is json or in the usual userPrefs.js sort of form.

justification

• Especially during development, having prefs control some aspects of an addons behaviour is useful, such "theaddon.demoMode=true" or Logging prefs, or "don't phone home" and many other cases.
• it should be EASY to override "just a few" prefs, without having to go through all the hassle of profile creation and copying.

Simplificiations / Alternatives

• only allow json

Prior art

• cfx, jpm
• shield does an enhanced form, where it has special shorthands for "the prefs tree for this addon"

[rpl]

@gregglind we recently introduced a similar feature in #658 :

web-ext run --pref "a.custom.pref.name=value" --pref "othe.custom.pref.name=value"

it is not exactly the same as the one described above, mostly because it would be tedious to pass many customized preferences (but it could be easier when once we have added the support for a user-level and project-level config file as part of #176).

How it sounds to you?

If #658 and #176 are not enough to solve this issue, we will be absolutely more than happy to discuss about what is missing and how we can achieve it.

[kumar303]

I think #658, #176, and #123 will address this. Feel free to let me know if I'm missing something.