Now suddenly my office pc has had the bookmarks taken for ransom

Was this someone you hired? A scammer?

Let's first make sure your system is clean:

You may have ad/mal-ware. Further information can be found in this article;
https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no

Run most or all of the listed malware scanners. Each works differently.
If one program misses something, another may pick it up.

Firefox automatically makes backups of the bookmarks. These are stored in the bookmarkbackups folder.

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 Restore. From the list, pick the file you want and click it.

The backups are stored in a jsonlz4 file named something like this;
bookmarks-2016-06-29_1424_enD1GTH ya0JmicnYWg4V4g==.jsonlz4

In this example; 2016-06-29 = date created, 1424 = number of entries.

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

https://support.mozilla.org/kb/forum-response-recover-bookmarks-after-update

If I don't want the same Firefox bookmarks on every device, do I need to create different accounts for each device using Firefox?

Hello   KatKan,

No, you don't have to create a different account for every device;
you can exclude bookmarks from Sync :

Type in the address bar   about:config   and press Enter.
(ignore the warning)

Type in the search bar and look for the preference :

services.sync.engine.bookmarks

and set its value to   false.

Then close and restart Firefox.

(edit : typo)

Note that you can also set what to sync via "Options/Preferences -> Sync" or via the Firefox Account toolbar button drop-down list (Sync Settings).

• https://support.mozilla.org/en-US/kb/how-do-i-choose-what-types-information-sync-firefox