Can I upgrade my iMac desktop from Sierra?

It can be upgraded to High Sierra but not beyond that. See the following from Identify your iMac model - Apple Support

iMac (21.5-inch, Mid 2011)

Model Identifier: iMac12,1

Part Number: MC309xx/A, MC812xx/A

Newest compatible operating system: macOS High Sierra

To upgrade, see instructions in: How to download and install macOS - Apple Support

Be sure to have a current backup first.

It could also be an iMac (21.5-inch, Late 2011). Apple gave the Mid and Late 2011 21.5-inch iMacs the same model identifier. The newest compatible operating system is High Sierra, either way.

High Sierra will not be enough to let the OP install the latest version of Chrome. That requires macOS 11 (Big Sur) or later. I could be mistaken, but I believe that Google does not provide old versions of Chrome for download – so even if there was an old version of Chrome, which ran on High Sierra, which was a bit newer than the version that the OP has, the OP could not install it.

There is FireFox 115 ESR. It doesn't get new features, and Mozilla had been planning to stop offering even "critical security updates" for it by last September or October. But it got a slight reprieve and will now get security updates through March 2025. It supports Sierra, so the OP could use it even without upgrading macOS.

Mozilla Support – Firefox users on macOS 10.12, 10.13 and 10.14 moving to Extended Support Release

Yes - you can go to High Sierra, and then download the GitHub project Firefox Dynasty which is based on Firefox 134 (and at the moment is being updated whenever a newer version of Firefox comes out). I probably aren’t allowed to post the link here but if you go to GitHub then search for either the creator i3roly, or for Firefox Dynasty, you will find the relevant page.

Firefox 115 ESR is being supported by Google until March, but not after. Obviously it will carry on working after then but will gradually degrade.