Bookmarks are stored in the places.sqlite file in your Profile folder.
https://support.mozilla.com/en-US/kb/Profiles#w_how-do-i-find-my-profile

I'm using Windows Vista Home Premium, when I'm at step 1, when I click on the "Start" button, I do not see a search box at all at the bottom of the "Start" button. Therefore, it's impossible for me to enter the string "%APPDATA%...".

In this sense, my problem is not yet solved.

Can you please send me instructions with simple, exact, and explicit steps as how I can get to my Mozilla Firefox profile?

Please get back to me with new instructions.

Thank you very much for your help, but it does not help at all.