The 'find in page' bar at the bottom of the page, invoked by Edit>find.
How can I keep this bar up/present continuously?
Once the Find Bar is opened, it will remain open for the current session, unless you close it. If you want the Find Bar to be open when you start Firefox see: https://support.mozilla.com/en-US/questions/773700
If the above link does not answer your question, you will need to provide more details concerning
I personally use this the following addon: https://addons.mozilla.org/en-US/firefox/addon/findbar-tweak/