browser.pagethumbnails.capturing_disabled

[SINE]

You have set
user_pref("browser.pagethumbnails.capturing_disabled", false);

This means the feature is active.
Mozillas wiki page regarding that setting is contradictory, quote:

Default value: true

Values
false (default)
The application creates screenshots of visited web pages.
true
The application doesn't create screenshots of visited web pages.

Given the name of the setting it must be assumed that setting it to true means the feature is turned off.

[kaihendry]

I hate these contradictory _disabled preferences. Wonder if it's worth filing a bug on Mozilla about that horrible practice.

[pyllyukko]

Thanks for spotting this.