It seems like some have changed the theme of your browser. Anyway, I would like to suggest that you change your browser’s theme back to default or anyone else. Apply the following steps:
Steps:
Open your browser (Firefox).
Go to menu/options iconĀ (three lines icon).
Click on Settings.
Click on “Extensions & Settings” from the left side pane at the bottom.
Now, click on the themes from the left side pane on the top.
Now, select the “System theme (Auto)” and click enable button.
If the problem still persists then choose any other theme of your choice and enable it.