.json files need to be restored, not "imported". "Import" is for bookmarks.html files that were exported.

https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them

You can check the permissions and ownership of these files via the right-click context menu (Properties).

Sadly this does not fix the issue. just for abit of context, the old PC is a windows xp while my current one is running windows 10 if that helps.

These instructions are for Firefox. Just do the same with other browsers.

Open the Bookmarks Manager; Press the Alt or F10 key to bring up the toolbar, and select Bookmarks. Hot key is <Control> (Mac=<Command>) <Shift> B.

Once the window is open, at the top of the page, press the button labeled Import and Backup. Select Export Bookmarks To HTML, and follow the prompts and save it to a HTML file. Copy the file to another computer/profile. Repeat the instructions above, BUT select Import Bookmarks From HTML.

https://support.mozilla.org/en-US/kb/restore-bookmarks-from-backup-or-move-them

https://support.mozilla.org/en-US/kb/recover-lost-or-missing-bookmarks