I'll probably start using bookmarks instead of tabs in future for loading previous sessions to be on on the safe side or is the Bookmarks file also vulnerable to corruption ?

Well... bookmarks and history are stored together in a database file named places.sqlite. Users report this file becoming corrupted occasionally; it seems to happen most often with a power cut. Fortunately, Firefox creates bookmark backup files on an approximately daily basis, for up to about two weeks. These should appear on the Restore menu in the Library window, as described in this article:

Restore bookmarks from backup or move them to another computer

You can increase the number of backups if you like. Here's how:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste max_back and pause while the list is filtered

(3) Double-click the browser.bookmarks.max_backups preference to display an editing field, and increase the value from 15 to whatever you like, then press Enter or click the blue check mark button to save the change.

More info on about:config: Configuration Editor for Firefox.

An update - this is semi related so I wasn't sure if I should start a new topic or not so I'm posting here

I recently noticed that all my history older than a month was deleted which went back 2 + years Does Firefox have a limit to how much history can be stored  ? Same question for number of Bookmarks There was a recent power outage but Firefox wasn't running when it happened

I'm confused - You mentioned that Bookmarks are stored in places.sqlite but in the link you gave it says they are stored in bookmarks-"date".json  ? Can either file be used for backups ?

(3) Double-click the browser.bookmarks.max_backups preference to display an editing field, and increase the value from 15 to whatever you like, then press Enter or click the blue check mark button to save the change.

Does the 15 refer to the number of backups or the the intervals with which they are made ?

I recently noticed that all my history older than a month was deleted which went back 2 + years Does Firefox have a limit to how much history can be stored  ?

Firefox computes a limit based on disk space. Unless your drive capacity suddenly went way down, I don't know why it would get trimmed so severely.

Same question for number of Bookmarks

I'm not aware of a limit.

I'm confused - You mentioned that Bookmarks are stored in places.sqlite but in the link you gave it says they are stored in bookmarks-"date".json  ? Can either file be used for backups ?

places.sqlite is the live file that stores bookmarks and history together. The files in bookmarkbackups are snapshots that can be restored and have bookmarks only. If you want to back up history, places.sqlite would be needed. For bookmarks only, that may be overkill.

(3) Double-click the browser.bookmarks.max_backups preference to display an editing field, and increase the value from 15 to whatever you like, then press Enter or click the blue check mark button to save the change. Does the 15 refer to the number of backups or the the intervals with which they are made ?

It should refer to the number of backups, and the interval is approximately daily, although days may be skipped if there are no updates. Firefox also may rename the file in the case that there's no change, so if it was the same on the 1st and the 2nd, the date in the file name may be the 2nd while the date the file was created was the 1st. I haven't studied that in much detail,though.

OK a few weeks later I now notice that my history older than a month seems to get deleted

I checked the places.sqlite file and it's 105 Mb

The space on my hard drive fluctuates but it never goes below 5GB - my computer gives a warning it if gets too low anyway

It should refer to the number of backups, and the interval is approximately daily, although days may be skipped if there are no updates. Firefox also may rename the file in the case that there's no change, so if it was the same on the 1st and the 2nd, the date in the file name may be the 2nd while the date the file was created was the 1st. I haven't studied that in much detail,though.

I still am a bit confused - You said 15 backups but there are only ever 4-5 maximum -

previous.jsonlz4 , recovery.baklz4 , recovery.jsonlz4 and upgrade.jsonlz4-20201211215739 in the sessionstore-backups folder and the temporary session restore in the main profile folder

I still am a bit confused - You said 15 backups but there are only ever 4-5 maximum - previous.jsonlz4 , recovery.baklz4 , recovery.jsonlz4 and upgrade.jsonlz4-20201211215739 in the sessionstore-backups folder and the temporary session restore in the main profile folder

15 was bookmark backups, not session history file backups.

OK a further update

Now my history doesn't get saved beyond the last 7 days As I mentioned previously my Bookmarks file is very large and has only increased since then, so is it possible that they are 'competing' for resources i.e the more Bookmarks that get added the less History gets saved ?

Now my history doesn't get saved beyond the last 7 days As I mentioned previously my Bookmarks file is very large and has only increased since then, so is it possible that they are 'competing' for resources i.e the more Bookmarks that get added the less History gets saved ?

Firefox doesn't have a date-based cut-off on history. Firefox has a maximum number of places it will track in the database file which has your bookmarks and history. Firefox recomputes this limit at some interval (I don't remember) and which may be reduced over time if there is less disk space available. You can check the current value in about:config under places.history.expiration.transient_current_max_pages -- usually it is over 100,000. Is yours much lower?