Short Version : Move on and Let It Go
I've recently left a company for whom I developed an open source project for over two years. The project is now gaining traction with its intended community, so in many ways my hard work is paying off.
You were paid, you left, you're not being paid to do it now.
Time to move on to new pastures.
Of course, with more users means more bug reports, feature requests, and contributions. Because the project has been put on the back burner by the company, most of these issues go unanswered for months. It pains me to see this and I would like to keep contributing to something in which I still see merit.
This is a psychological issue. You feel a sense of responsibility and ownership towards the project you worked hard on. However you did not own it and your responsibility de facto ended when you left.
You're having trouble letting go, but in the course of your career you'll find you have to leave maybe hundreds of projects you invested time in (and got paid for !) and they'll all need more work. You can't keep them all.
My question is: Should I keep contributing to an open source project which belongs to a company I no longer work for, and if so how?
No you should not.
It's not practical and you are working for free. Never work on what is in fact a commercial project very loosely pretending to be open source.
Naturally, I am no longer an admin on the project so I can't merge/close PRs. I'm reluctant to answer issues directly as it might give the impression that I'm still part of the organization. I've considered forking it under my username and continuing development, but fear it would seem in bad faith (like I'm trying to "steal" their users).
If it's really open source then forking is something they have to cope with. There's no need for guilt.
Note that they would not be operating (or claiming to operate) an open source model of development unless it was beneficial to them. Companies get something from this process, even if it just makes them look "friendly" from the outside.
You can fork with a clear conscience if you really, really want to.
EDIT: All the repos are Apache 2.0 as of this moment. The project consists of a desktop application which contains the name of the company and two libraries which are branding agnostic. The company does not de facto accept pull request from third parties.
Not accepting pull requests from third parties suggests it's not really being done as a proper open source project (at least in my mind). While there's no obligation to accept third party contributions in your own branch, it's pointless operating "open source" if you just blanket ignore such contributions.
From a practical point of view this means you cannot continue contributing to that project. You can fork it and create your own version which will become a different project (as they won't merge your contributions anyway).
EDIT 2: I'm still very close with most of the people at the company and left on very good terms. Current maintainers still ask me technical questions about the project, which has lead to some tongue-in-cheek remarks from my former bosses about me "working for free". They aren't open to paying me part time to maintain the repo. The reason I feel compelled to keep working on this is because of the domain knowledge I've accumulated; it's likely no one else knows as much as I do.
You are a working developer who gets the money they need to eat, drink, be housed and clothed from coding. Keep that in mind.
While it's perfectly reasonable to maintain contact and answer the occasional technical question with colleagues (that's just networking - a good thing to do as long as it flows both ways), if you're really operating as an unpaid consultant then you need to put the brakes on this.
Don't drift into becoming an unpaid worker for your former employer.
You left the company and all the work it did behind. You need to accept this as a fact and move on mentally.