I need to download firefox and my computer is not letting me

The current version of Firefox (97) requires macOS Sierra (10.12.6) or later. The age of your Mac determines the operating system that you can upgrade too. From your Apple () menu > About This Mac, look at the product description line with parenthesis in it. Compare that string with the hardware requirements on How to get old versions of macOS. Use exclusively the Safari browser when accessing that link, and choose the operating system link to the Mac App Store where you hardware is supported.

Before you attempt any upgrade, backup your Mac to an external drive with Time Machine. For the operating system that you can upgrade to, check any third-party vendor software requirements to determine if they support it, and do the same for the printer/scanner vendor's drivers, as a precaution that they may have dropped driver support.

If you are upgrading to macOS Catalina or later, you are restricted to 64-bit applications, and 32-bit applications are unsupported.

Macs don't run iOS, they run macOS.

What version of macOS are you running? Exactly what model Mac do you have?

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence.

Computer Qualify for the Monterey upgrade

Empty Space Requirements  >> If upgrading from macOS Sierra or later, your Mac needs 26GB of available storage to upgrade. If upgrading from an earlier release, your Mac needs up to 44GB of available storage. 

macOS Big Sur 11 hardware requirements

Empty Space Requirements specific to the Big Sur. Apple states 35.6 GB Empty Space but suggest 50 GB

macOS Catalina 10.15 hardware requirements

macOS Mojave 10.14 hardware requirements

Caution - jumping over all the newer versions and directly to Big Sur  / Monterey on Qualifying computers is Technically possible - there are those who have attempted and were successful. Then there are those who attempted and had less than good results.

If this is a Production or Mission Critical Set-up - a more prudent and measure approach is suggested. 

The above is from observations on the Apple Support Community Forums and IMHO

Tips Big Sir Upgrade Released Nov 2020

Tips Monterey Upgrade  Released Oct 25, 2021

Make Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur & Monterey supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may required update or upGrades to 64 Bit Versions.

That's not helpful.

Please read this: Writing an effective Apple Support Communities question

If necessary, post in your native language.

Why not? What happens when you try?