All media 404 in Firefox

### Steps to reproduce

Accessing via https://app.element.io, on my homeserver running latest synapse.

Scrolling back up through a chat with media (images/videos) in Firefox all errors with "Unable to show image due to error". Videos fail to play.

In the browser network tab, these requests show 404 responses.

No such 404s are observed in Chromium, the media loads fine there. Same in Element X mobile app.

### Outcome

#### What did you expect?

Expected media to load

#### What happened instead?

Media didn't load

### Operating system

Linux

### Browser information

Firefox 153

### URL for webapp

app.element.io

### Application version

Element 1.12.26

### Homeserver

synapse 1.156.0

### Will you send logs?

Yes

Start by reproducing the failure at app.element.io in Firefox 153 and inspect the media requests in the browser network tab, comparing them with Chromium where media loads correctly. Trace the web app's media loading path from the failing requests; done means images and videos load in Firefox without 404 responses.

Written by the indexing model from the issue text.