Does this mean that so long as you are logged in only for as long as necessary, and don't do anything else on the computer at the same time, you're reasonably safe? Also, is this something that you could block with a firewall?
It depends what you mean by 'safe'.
In terms of a corporation using the data you've uploaded for a purpose you wouldn't want them to, then my own view is the risk is low - not because I trust the corporations, but because of the vast quantity of data and the utter irrelevance of most of it to the corporations concerned means that the reward vs effort equation isn't that positive for them.
Nevertheless, I personally wouldn't upload anything containing my full name, phone numbers, email, or postal address. In fact nothing I wouldn't be comfortable putting in the recycling/rubbish bins outside my house. The reason for that is that once something is out of your control, you have no control over what other people might do with it.
The second point is that what is 'safe' depends entirely on what permissions and cookies etc you have allowed on your computer and accounts. Tracking and monitoring are now so pervasive that anything you do on an internet-connected computer
could be recorded and (later) added to a corporate database, even if you are logged out / disconnected from the internet at the time you are doing it.
A firewall makes no difference if you have given consent (even unknowingly) to having your data and activity tracked and logged.
To be really 'safe' requires a level of knowledge and dedication that few of us have. That's why I gave up a long time ago, and have maintained a second (or more!) computer which contains all my 'personal' information, and which
never goes online. It also means I could stick with Windows XP on that machine without fearing the doomsday scenarios used to encourage us to update/upgrade yet again.
Without the option of a second computer, I think by now I'd be using only a version of Linux developed by people who do have the knowledge and dedication required to make life as difficult as possible for the corporations. There is always a trade-off between security and convenience, but on balance I'd normally go for the security of Linux vs the convenience of MS or Apple products.