After further investigation, it seems to be linked to whether or not there are explicit width and height properties on the svg. When missing, the error is triggered.
Workaround is to ensure there are explicit widthandheight properties on your svg.
When either is missing then that dimension is registered as 0px and results in a 400 error when trying to save the document. Would be really helpful if Prismic could add in a more helpful error here, even saying which field was preventing the save would have saved me a lot of time.
Welcome to the Prismic community,
Thanks for sharing the workaround but in order to investigate this issue, can you please share with us the name of your repository (in a private message if necessary).
As well as the SVG you are trying to upload?
It would be also useful if you can add a screenshot of the error you get, and if you are a developer the error you get in your browser console.
So to summarise: SVGs without width/height properties when uploaded through Firefox cannot be saved in documents (in any browser). So there appears to be an issue with the file upload process.