[current] Expose a couple useful hidden prefs in about:config by default

[laniakea64]

privacy.firstparty.isolate.block_post_message
Fixes #1081

privacy.resistFingerprinting.block_mozAddonManager
This does also change the default value of this pref as per #204 (comment) . Not locking this pref since I've occasionally found it useful to temporarily flip it to false.

[MrAlex94]

Cool, thank you.

[grahamperrin]

privacy.resistFingerprinting.block_mozAddonManager

From https://www.reddit.com/r/waterfox/comments/bt4nm7/increasing_the_adaptability_of_addonsmozillaorg/ese7s24/

Downside

If the preference is true then you lose the ability to tell whether an extension is installed …

From #204 (comment):

… Just to point out, Waterfox 56 has this pref too - #449

Never seen by me on FreeBSD-CURRENT.

Build 20191210201058:

What's true for Waterfox Classic on FreeBSD-CURRENT is true for Firefox 71.0 on FreeBSD-CURRENT.

If the preference then true, then AMO loses functionality:

When false (or not set):

[laniakea64]

Downside

If the preference is true then you lose the ability to tell whether an extension is installed …

@grahamperrin Yep, that's exactly why I've "occasionally found it useful to temporarily flip it to false."

Speaking as someone who has made this a conscious choice, and at times chosen in favor of the AMO feature, I figure the emphasis on "occasionally" made clear enough that the downsides of setting the pref to true don't make AMO less usable. (Especially since the option remains available.)

… Just to point out, Waterfox 56 has this pref too - #449

Never seen by me on FreeBSD-CURRENT.
...
What's true for Waterfox Classic on FreeBSD-CURRENT is true for Firefox 71.0 on FreeBSD-CURRENT.
...

😕 Sounds like what you're seeing is that Waterfox Classic does have this pref, Waterfox Classic just doesn't expose it in about:config by default.

[grahamperrin]

Waterfox Classic just doesn't expose it in about:config by default.

Ah, thanks for the explanation.