Based on the telemetry data they’ve received on use of Fonts View in developer tools, Mozilla has disabled the feature by default in Firefox 47, but after receiving complaints from users, the company has changed its mind and re-enabled the Font Panel in Firefox 50 Nightly developer tools and it has been moved to the last position in the sidebar (check the below screenshot and this bug for more details). This will be uploaded to Firefox 48 beta 6 and aurora, we’re not sure about release.

Mozilla re-enables Fonts View by default in Firefox 50

Right now, Firefox 47 users on release/stable channel, can enable Font View manually by changing devtools.fontinspector.enabled preference value to true in about:config from (default) ‘False’  .

Fonts view in Firefox 50

READ: Firefox to offer Undo functionality for imported browser data