Add note for return value for webRequest.getAllFrames on Firefox. (#10492)
Add note for return value for webRequest.getAllFrames on Firefox. (#10492)

* Add note for return value for getAllFrames on Firefox.

* Apply changes from review

Co-authored-by: Daniel D. Beck <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@ddbeck.com>
mdn/browser-compat-data,mdn/browser-compat-data,chrisdavidmills/browser-compat-data,chrisdavidmills/browser-compat-data