Find mode n and N do nothing in private windows (Firefox, Ubuntu)

[ghost]

Describe the bug
After searching for text with /<some text><ENTER> n and N ought to jump to respectively the next and previous occurrences of <some text>. Instead, however, they do nothing.

To Reproduce
Steps to reproduce the behavior:

1. Go to URL example.com
2. Type /am<ENTER>
3. Type n
   What ought to happen: jump to the next occurrence of am
   What actually happens: nothing

The screenshot shows before and after step 3 (webpage is dark because of Dark Reader extension).

Browser and Vimium version

about:support
Name: Firefox
Version: 78.0.1
Build ID: 20200630195452
Distribution ID: canonical
User Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:78.0) Gecko/20100101 Firefox/78.0
OS: Linux 5.4.0-40-generic
Application Binary: /usr/lib/firefox/firefox

about:addons
Version: 1.66
Last Updated: 3 July 2020

[sid-maddy]

@Dante-Isler, do you face this issue in normal windows or only in private windows?

[philc]

This should be fixed in c859406.