Firefox/Mozilla can't support older OS forever because of the security holes they present currently. One can't support outdated OS forever. I use Windows 10 and this works fine currently. You can have the older Windows OS just that you can no longer be safe and security when connected to the internet so it will be Offline computer going forward. I've loaded W10 on computers that had Vista and Windows 7 and they runs just fine in Windows 10.

https://support.mozilla.org/en-US/kb/firefox-users-windows-7-8-and-81-moving-extended-support

Firefox 115 ESR is currently at Fx 115.18.0esr and is planned to have updates until at least Fx 115.21.0esr in March 2025 unless Mozilla decides in early 2025 on whether to extend updates again as Fx 115 ESR was originally going to stop at Fx 115.15.0esr (released September 3, 2024).

Other major web browsers have already dropped support of the EOL Windows 7, 8, 8.1 back in Jan/Feb of 2023 so Mozilla will have been supporting these three old versions of Windows for over two years longer at minimum.

You can keep on using Firefox 115.21.0esr if that ends up being the last update however it will then no longer get anymore security updates. Though it is possible if there is some serious critical security concerns that they may put an update out then.

Also keep in mind that Firefox 115 ESR is based on the old Firefox 115 Release and over time it may not work as well with some websites that work well in more current Firefox versions.

The Firefox ESR short for Extended Support Release have been meant more for Enterprise users in mind as they like to have long term stability and may even mean sometimes not all security fixes gets put in to keep it stable.

However Mozilla have used ESR versions to support EOL OS's for a lot longer compared to other major web browsers. Firefox 78.15.0esr was the last to support the macOS 10.9, 10.12, 10.11 and earlier on Mozilla supported the then WinXP/Vista with Firefox 52 ESR (last was 52.9.0esr) for over 26 months after others dropped support on XinXP/Vista.

So despite Mozilla supporting EOL OS's for over two years longer, some people think it would be easy for Mozilla to spend money and resources (that they can use for more current things) to continue supporting EOL OS's for even longer.

Besides using Windows 10/11 the other option is to dual boot with a light Linux distro with a desktop like say XFCE or MATE as avoid the heavier KDE and GNOME in order to use current versions of Firefox

edit: from https://whattrainisitnow.com/release/?version=esr

We decided to extend support for ESR 115 only on Windows 7-8.1 and macOS 10.12-10.14 up to March 2025. We will re-evaluate this decision in early 2025 and announce any updates on ESR 115's end-of-life then.

Thanks for the response James, but I'm still unclear on the matter - yes, I'm that dense I'm afraid when it comes to current computing matters, given that I always have like 10-15 YEARS gap between having/using computers (this is my 2nd computer and its already 10yrs old after like 15yrs of not having any!) and I really have no one else I can ask so I hope you'll forgive my ignorance. Anyway, I'd like to clarify a few details. If I understand right I can still use Firefox (up to the 115.21.0esr version, if that is the last one like you say) with Windows 8 beyond the March 14, 2025 expiry date only I won't have security updates (I wonder if the antivirus will kick in then to protect). What I would like to know is whether this will affect my extensions (the above mentioned ad blockers, ghostery and videodownloader - as well as the themes) when they want to update after March 14, 2025 (and in case I would need to get back to a previous version do you know how to do so? I've tried once and could not) or might it be best to update them till March 14, 2025 and not beyond? Also, you mentioned something about a dual boot with a light Linux distro with a desktop like say XFCE or MATE as avoid the heavier KDE and GNOME to use with the current Firefox. I must say I have no idea what all of this means (and searching google will not help I'm sure - I've tried before with easier stuff). What is XFCE or MATE? Anyhow, I appreciate the effort. As to Firefox having extended its support to old OS longer than others I also appreciate but the fact remains that there are still a bunch of us poor people who, for a reason or another, cannot get a new computer or update to later OS - like me, I won't even attempt it alone given that I'm not sure how to go about it and after having witnessed and read of others more knowledgeable than me having met with disaster and going back to the older OS to simply have back a functional computer was, to say the least, an hard task that I have no idea how they managed it (they don't go in details or else is in some code you people understand but I cannot follow - sorry!). What I was pleading is for Firefox to give us also a bit of an helping hand. I don't mean they have to spend a lot of time and energy and money for us, but couldn't they keep us in the loop like and offer their support even if is not as constant as for the newer versions for the latest OS? Could they not follow the Big Corporations (awful) treatment of people? I know it's asking a lot and Firefox cannot help for ever but like I said we are not all on the same technologically advanced page for whatever reason and though it's up to us to better our situations sometimes it's simply not possible - and for me at least right now Internet has become a life-line that is why I'm pleading so. Anyway, sorry for the commentary and thanks for the answer - and if you could clarify about my doubts above I would appreciate it. Thanks.

Linux distros being like the whole list of options at https://distrowatch.com/ . Now days they are free to download and use though there are few that are paid for but not really any better as a result Some may just have been paid for if only to get some direct support. Though perhaps they may be out of your comfort level in deciding on one to use, installing and using..

As I mentioned the older Firefox 115 ESR was originally planned to end as of Firefox 115.15.0esr (released September 3, 2024) but they on almost last minute revealed they decided to extend support to Firefox 115.21.0esr. I do believe that if they were to decide to extend updates support again, I would not be surprised if it is only for another six months of updates at most as this current updates period is six months.