Mild tomatillo salsa - can I can?
You could certainly freeze it in any case. It will be safe in your freezer more or less indefinitely.
If the pH is below 4.6, you could can in water. This is probably the case, but it's hard to say for sure unless you print the recipe (that you follow to a T) or test it.
If the pH above 4.6 (or even if it's particularly close, if you tested at home), then pressure canning is the only way to go.
...or if it's particularly close, you can just add more acid. Either more lemon if there is room in the flavor profile for it, or some citric acid which has a more neutral flavor and tends to hide in the background more.
But pressure canners are great to have around anyways to do stocks, chilis, salmon in season etc. with at home. Leaves more freezer space for other stuff.
I have used the one from the Ball book if you have it. Not sure if this is the exact recipie, but here is an option.
PS - I have also frozen it which works just as well if you have the option, but my personal preference is canning, and it is easy...