Older operating system - browsers are all unsupported

You are not "being forced into buying a new computer". You simply have not taken full advantage of the software upgrades that are available to you right now.

The operating system of your MacBook Pro 13-inch 2015 can be upgraded to macOS 12 Monterey.

MacOS 10.12 Sierra is lagging seven (!) generations behind the currently shipping macOS 14.

Upgrading to macOS 12 Monterey can buy you several more years of supported browser use with Safari, Firefox, Brave and others.

If you don't upgrade the OS to something newer, then you are cutting yourself out of the longest possible productive use of your Mac.

As Limnos has posted above, upgrading from macOS 10.12 Sierra to macOS 12 Monterey may require that many of your apps and hardware drivers also be upgraded because the older ones may only be written with 32-bit code. Apps coded in 64-bits will be required under macOS 10.15 Catalina and newer, including macOS 12 Monterey.

You can download the free utility Go64 from St. Claire Software to check the 64-bit compatibility of your currently installed apps. Once you know, then you can better plan your upgrade.

FYI -

✅ Check the compatibility of your Mac hardware with newer versions of macOS software:

➡️ macOS 10.13 High Sierra is compatible with the Macs listed here (c.2009)

➡️ macOS 10.14 Mojave is compatible with the Macs listed here (c.2012)

➡️ macOS 10.15 Catalina is compatible with the Macs listed here (c.2012)

➡️ macOS 11 Big Sur is compatible with the Macs listed here (c.2013)

➡️ macOS 12 Monterey is compatible with the Macs listed here (c.2015)

Hi,

Also, while investigating the available macOS upgrade options mentioned by Limnos and D.I. Johnson, you may want to have a look at the following links (Mozilla offers extended support until September 2024 for Firefox version 115, which possibly could be of help as a temporary solution in the meantime):

https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support#firefox:mac:fx115

https://www.mozilla.org/en-US/firefox/all/#product-desktop-esr

With a 2015 Mac you can run higher than Sierra. I just installed Big Sur on a 2013 MacBook Pro. You'll need to upgrade if you want a newer browser.

The various ways of checking for software may only show upDATES to the current main system version you are running (e.g., 10.4.8 --> 10.4.11), not upGRADES to a higher system version such as a jump from El Capitan (10.11) to Catalina (10.15). Note the difference between an update and an upgrade.

Upgrading a macOS depends upon the model Mac, specifications, and year. To get more information about your computer, choose:  (Apple menu) > 'About This Mac' in the upper left corner of any window, then "More Info..." or "System Report". There's more about this in "About System Information [Profiler] on your Mac" - System Information User Guide for Mac - Apple Support

Use the computer information, including the model year, and the links below to determine what is possible with your computer.

Click on this link to read how to get different versions of macOS --> How to download and install macOS - Apple Support

- The article also has links to system requirements for each OS version which you should check against your exact computer model. For any model produced in the last 5 years or so you should use the system requirements links for each OS version instead. This web site also has a reliable presentation of model and OS version compatibility: https://eshop.macsales.com/guides/Mac_OS_X_Compatibility

- With an old computer it may help to have more RAM (memory) than what is noted as "minimum" for any particular system or your computer may run slowly.

- You may find you must use Safari to get links for High Sierra and later using the Mac App Store application to open to a functioning page.

- Sierra, El Capitan, and Yosemite download as a .pkg. Click on this to have a converted version placed in your Applications folder as an "Install OS..." application. Run the installer and follow the directions.

- Upgrades may take some time (up to an hour or more) to install, with restarts and blank screens.

- If you encounter download problems it may help to download while in Safe Mode Use safe mode on your Mac - Apple Support

Before upgrading, back up at least your data in case something goes wrong. See this support document: How to back up your Mac - How to back up your Mac - Official Apple Support

Check if your old apps will still work with with the OS you want to install - https://roaringapps.com/apps This is particularly critical if you are upgrading to Catalina or newer, as older 32 bit apps will no longer work. To check for those, try this 32 bit application scanner application --> https://www.stclairsoft.com/Go64/