Running multiple instances of RStudio in Docker

Question

I'm running multiple instances of RStudio on different ports:

In one terminal

docker run -e PASSWORD=yourpassword --rm -p 8787:8787 rocker/rstudio

In a second terminal

docker run -e PASSWORD=yourpassword --rm -p 9090:8787 rocker/rstudio

If I access these sessions (http://localhost:8787/ and http://localhost:9090/) from the same webbrowser (e.g. firefox) in different windows, the pages reload constantly (every ~1.5 seconds) and prevent me from using RStudio. If I use two different browsers (e.g. firefox and chrome), the constant reloading stops and I can use RStudio.

What's the underlying problem here? How can I open both instances from the same webbrowser?

PS: The constant reloading of the pages also happens if I open both pages with chrome.