"Content-Length required" error in Firefox when saving preferences
When attempting to save preferences or enable HLS playback on Firefox, I get a blank page with a "Content-Length required" error.

Request:
```
POST /saveprefs HTTP/1.1
Host: nitter.net
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:91.0) Gecko/20100101 Firefox/91.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br
Content-Type: application/x-www-form-urlencoded
Content-Length: 179
Origin: https://nitter.net
DNT: 1
Connection: keep-alive
Referer: https://nitter.net/settings?referer=%2F
Upgrade-Insecure-Requests: 1
Sec-Fetch-Dest: document
Sec-Fetch-Mode: navigate
Sec-Fetch-Site: same-origin
Sec-Fetch-User: ?1
Pragma: no-cache
Cache-Control: no-cache
```

Response:
```
HTTP/2 411 Length Required
server: Caddy
date: Wed, 25 Aug 2021 16:04:33 GMT
content-type: text/plain; charset=utf-8
X-Firefox-Spdy: h2
```