Lags when scrolling a page in Firefox.
Scrolling with a delay.
Firefox with the Dark Backgraund Extension. On half of the websites the font breaks.
Your questions can be answered as per: https://whonix.org/wiki/Support#Free_Support_Principle
- To start with try to remove the Whonix specific part of the question. These issues have to do with Firefox web browser.
- Using a search engine look for “Firefox scrolling lag” and “Firefox dark background breaks fonts”
Also you should not be using any browser other that Tor Browser
Warning: Only Tor Browser is recommended for use in Whonix when browsing the Internet.
Warning: Using regular browsers is pseudonymous rather than anonymous.
If browsers other than Tor Browser are used in Whonix, the user’s IP address and Domain Name Service (DNS) requests  are still protected. However, users do not profit from Tor Browser’s protocol level cleanup in this scenario. Features like proxy obedience, state separation, network isolation, anonymity set preservation and a host of others are simply unsupported by other browsers.
In stark contrast to regular browsers, Tor Browser is optimized for anonymity and has a plethora of privacy-enhancing patches and add-ons.  With Tor Browser, the user “blends in” and shares the Fingerprint of nearly three million other users, which is advantageous for privacy.