Let's consider those independently:

Cache - No effect, feel free to clear cache. This article lists one method: How to clear the Firefox cache.

Cookies - May remove cookies stored in your session history file, meaning that when tabs are restored, you are not logged into those sites, but shouldn't remove any tabs from the list.

History - Open tabs are considered part of your history, so setting Firefox to delete browsing history at shutdown -- using the Clear history when Firefox closes feature on the Options page -- will make it impossible to restore your previous session.

On the other hand, clearing history while those tabs are still open -- using the Clear Recent History feature -- shouldn't close the open tabs. I suspect Firefox will still keep a session history file of the open tabs in that case to protect against a possible crash, but I have never experimented with it... Hopefully someone else can give you a more definitive answer.

If you are thinking of experimenting, you might want to back up one or more of your current session history files. To do that:

(1) To open your profile folder...

With Firefox running, you can open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, click the "Show Folder" button.

(2) Copy out session history files

In your profile folder, scroll down and double-click into the sessionstore-backups folder. Save recent files here to a safe location, such as your Documents folder. The kinds of files you may find among your sessionstore files are:

• recovery.js: the windows and tabs in your currently live Firefox session (or, if Firefox crashed at the last shutdown and is still closed, your last session)
• recovery.bak: a backup copy of recovery.js
• previous.js: the windows and tabs in your last Firefox session
• upgrade.js-build_id: the windows and tabs in the Firefox session that was live at the time of your last update

Note: By default, Windows hides the .js extension. To ensure that you are looking at the files I mentioned, you may want to turn off that feature. This article has the steps: http://windows.microsoft.com/en-us/windows/show-hide-file-name-extensions

Great, Thank you