As has been indicated above, a 'normal' uninstall of Firefox (using the Add/Remove Programs feature within Firefox) usually seems to leave the users profile still stored on the computer.
A 'deep' uninstall (which is probably what Revo does) removes everything.
To see if there's anything useful left, your grandson should do the following:
Click the Windows Explorer button (next to, or close to, the Start button)
Then go to Organize > Folder & Search Options > View
Put a check mark against 'Show hidden files, folders and drives'.
Click 'Apply' and 'OK'
Click on 'Windows(C)' in the left panel.
Then double-click on each of the following in turn:
Users
[His username on that computer]
AppData
Roaming
Mozilla
Firefox
Profiles (the folder with that name, not the other 'Profiles' entry)
At that stage he should be hoping to see TWO folders with long names (such as yrj94eht.default-1485803867831). If so, he's in the luck because the older folder is his original profile. All he needs to do then, in order to get his bookmarks back, is to open the older folder, copy the file called places.sqlite (which is the one with his bookmarks in), go the newer folder and paste the old file there (replacing the newer version of it).
If there's only ONE folder with a long name he's probably out of luck. However it might still be worth looking in that folder to see if there are any back-ups of his bookmarks (although I doubt that there will be). To do so,he should double-click on the folder and then on bookmarkbackups. If there's a suitable backup in there he should be able to use the instructions here:
https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them#w_restoring-from-backups
If your grandson experience problems with Firefox in the future his first option (rather than uninstalling) should be to 'refresh' Firefox; that will retain his bookmarks but otherwise take Firefox back to its 'out of the box settings'. (To do so, click the three lines at the top right. Then click on Help > Troubleshooting Information > Refresh Firefox).
If he ever decides that he does want to uninstall and then reinstall Firefox then, as stated above, simply using the Add/Remove programs feature
might retain his old profile. However (irrespective of the method of uninstalling used) it would be best to export his bookmarks first so that, if required, they can be imported again later. To do so, press Crtl+Shift+B, then click on Import & Backup. Click on Export Bookmarks to HTML, select a suitable location for the file and save the backup there. (To import them back into Firefox, go through the same procedure but simply use 'Import' instead of 'Export').