As to which Browser Twitter now "X" accepts is their decision and Firefox being a open source Browser with community developer support and maybe it's not what "X" wants users to use to access their site. Maybe Firefox devs can change this in the future updates but if "X" choose not to allow Firefox to access their site there's not much Firefox can change that. I use Chrome and that may be what they want users to use on their site and so far haven't had issue accessing the site. But this is my own read from this it could be different but if "X" choose to do this it's up to "X" to change that as well.

As to which Browser Twitter now "X" accepts is their decision and Firefox being a open source Browser with community developer support and maybe it's not what "X" wants users to use to access their site. Maybe Firefox devs can change this in the future updates but if "X" choose not to allow Firefox to access their site there's not much Firefox can change that. I use Chrome and that may be what they want users to use on their site and so far haven't had issue accessing the site. But this is my own read from this it could be different but if "X" choose to do this it's up to "X" to change that as well.

Please see the attached image to see where 'X' themselves stipulate that Firefox continues to be a supported browser.

You've done all the right steps. I even checked your useragent, which tells sites what version of Firefox you have. And it is the latest one, 131. The only things I can of think you might need to do are clear all the cache in case some Twitter files are corrupted there. https://support.mozilla.org/en-US/kb/how-clear-firefox-cache#w_clear-the-cache

Good work using the Forget feature to completely forget Twitter. That should've deleted everything related to Twitter for a fresh start. And very smart adding Twitter to Exceptions list for Enhanced Tracking Protection. As Twitter was completely usable earlier this year after Firefox's Tracking Protection Strict mode and Twitter started fighting each other.

You might also want to try disabling your addons to see if any of them are accidentally breaking Twitter. I know it seems impossible but these days since websites are constantly tweaking themselves, new conflicts can emerge out of nowhere.

Read this article: Disable or remove Add-ons

Try disabling Block Twitter Promoted 0.1.18 and Good Twitter 2.6 first. Then close Firefox and reopen it. Go see if Twitter works now. If not, return to the addons area and disable ALL the addons. Even Adblock Plus. You just never know what could be responsible for these things. If that works, you need to turn on each addon one by one, while closing & reopening Firefox each time, and go back to Twitter and see if it gets rid of that "Browser is not supported" page.