I have worked on software for treating cancer, and I have worked for Mozilla. And boy, let me tell you, the latter sure felt like it had a much bigger impact on the world.
Let me tell you a little bit about medical software. It is incredibly hard and expensive to get data. It usually involves finding enough people who are dying in a particular way (already hard), and managing to collect comparable data from each (i.e., with the same process: even harder). There is never enough of it to do the statistics you want. Once you have some, and you maybe come up with some way to do some step of the treatment process slightly better, there are a lot of hurdles and it takes a very long time before anything you did can actually be used on a patient. Rightfully so. People's lives are at stake.
With cancer in particular, maybe after a decade in the field you might know if what you did had a positive impact on treatment outcomes. Maybe. That's the best-case scenario. For me, I don't even know if any line of code I ever wrote was actually used to treat a real patient. Oh, yeah, and those dying people whose data you used to come up with and test your idea? Yeah, you could not do anything for them. They got whatever treatments were the best available at the time, and good luck. If they were great treatments, you probably would not have been working on the problem. So that sucks.
Meanwhile, at a browser company, I was able to write a patch and within a month or three it was deployed to over a hundred million people. Software projects I worked on are currently used to encode and decode a non-trivial fraction of the bits on the internet. They are deployed on every Android handset and every iPhone (not just in Firefox), so that's a few billion devices. Every time you make a Zoom call or use probably almost any other video conferencing service, you are using at least one, if not several, of those projects. I understand people are doing a lot of video conferencing these days. Maybe even some telemedicine calls between radiation oncologists and their patients.
So, uh, yeah. I feel like that had more impact. Not to discourage anyone from trying to cure cancer. It's incredibly hard.
P.S., "cancer" is actually a thousand different diseases with a thousand different causes and requires many different treatments. There will never be a "cure for cancer".
Not to far down that list is probably Lotus Notes
Lotus Notes is awful. It’s Exchange, but worse. They wanted it to be an “open” platform for applications to be built on top of but failed to win over hearts and minds (and especially not hearts...).
If tomorrow we found the cure for cancer, and your work contributed even the slightest amount to it, that’s what you would be telling people at parties, not that you also worked at Mozilla (assuming you’ve done equivalent work in both fields).
Doing the work researching a cure for cancer is much more practical with 1 Mozilla.