riot.im/app causes firefox to show "a web page is slowing down your browser" warning

### Description
When opening https://riot.im/app (and /develop) firefox displays the "a web page is slowing down your browser, what would you like to do?" warning message on that tab, and sometimes other tabs as well. Selecting "stop it" results in riot being unusable until the tab is refreshes, and selecting "wait" just results in the warning being shown again after a few seconds.


### Steps to reproduce

Open https://riot.im/app in firefox

This does not seem to happen on other riot instances.

### Version information

- **Platform**: web
- **Browser**: Firefox 76.0.1
- **OS**: fedora workstation 32
- **URL**: riot.im/app