It may not be an issue of you using macOS but rather due to using the older Firefox 115 ESR which is based on the old Firefox 115 Released back on July 4, 2023 but with security and or stability fixes since. The recent Fx 115.33.0esr released on Feb 24 is the last planned major update as there will not be any more updates apart from perhaps any minor updates for security or stability fixes that warrants it.

If you are able to update your macOS to 10.15 (ten.fifteen) or later then you would be able to run Firefox 116.0 and later including the current Fx 148.0 Release and Fx 140.8.0esr.

Many websites have been require more current versions of web browsers and no longer support older Firefox versions like 115.

I cannot update MacOS above 10.13.6 because I cannot afford newer hardware at this time.