Peeps, really, I noticed this as well while checking for a coworker doing work on FF —I use FF personally but work in Chrome so I never noticed.
All the solutions presented in this and in previous replies, including the help page, seem to avoid the point that the feature works on people's machines in one browser but not in another.
I'm on a months-old machine that meets all the requirements and I've tested this, creating a new OSX user with clean installs of both FF Dev and Chrome Canary as well as the current regular versions with no addons and Firewall off, just to be sure —and the option isn't there in FF [screen].
You really shouldn't be suggesting reinstalls, firewall port settings changes, or Flash issues (Flash, of all things!) to clients who might be paying hundreds or thousands of dollars for the service, or deflect to 'desktop app' daemon updates on this issue without good reason and arguments. Nor should you close questions as solved when they clearly aren't at least one year on.
Maybe it is a Mojave issue, maybe a FF bug, but if x addon or y setting causes this, find it and name it and make sure it's the cause so visitors can enable/disable and see the change work. This is exceedinly frustrating and honestly not very trust-inspiring, esp. looking back at posts dating from early 2018 and before. 'Have you tried turning it off and on' is a joke for very good reason and users expect more from a paid professional service.
I'm fully aware how browser/OS version proliferation is a development pain, and how 'ghost bugs' might persist for no apparent reason on seemingly random setups but it wouldn't hurt acknowledging this yourself. If the questions keep reappearing, the issue is clearly not gone and the proposed solutions clearly aren't fundamental.
I'm sorry, but this is very disappointing and to be honest somewhat of a recurring theme with Dropbox.
[UPDATE] Noticed I'm on the Forum here not Support so no criticism intended towards fellow users trying to help; the question has been asked in multiple instances so this response is definitely intended for Dropbox reps and support staff. An issue like this really shouldn't even be left to community efforts to resolve.