Is this guide under construction, or is this really all there is? I was hoping for something slightly more detailed than just “Install Firmware.” Like, maybe, some steps to follow, so I know I’m not going to fry anything? https://ohai.lulzbot.com/project/cura-firmware-flashing/firmware-flashing/
Do you mean installing an own Marlin like RCBugfix or updating to a newer Lulzbot-FW release? For second, Cura will ask you on it’s own when a new version is available and do all needed steps alone.
I wanted to update my TAZ 5 firmware with the corrected 85deg print bed temperature setting. Maybe I misunderstood the message in that thread that said “Firmware is here.” Looking at it now, it may be just giving someone the URL to the firmware in general instead of announcing that the firmware has been updated.
I haven’t figured out yet what “Marlin” is. 
Marlin is the Firmware of your printer. The Lulzbot guys take it, configure it with the machine specific parameters (steps/mm etc), add the Lulzbot Logo and the result is what you call the TAZ 5 firmware.
OK then let’s wait for somebody more familiar with the procedure of Cura FW upload. I’m not using Cura or the Lulzbot Marlin releases, so I’m out 
I thought Cura is searching for updates on it’s own and a message will appear where you only have to hit OK.
You shouldn’t have to update the firmware for anything related to the bed. Just start with 105 degrees and raise or lower as you are able. Many people find tube sweet spot with a taz to be near 110.
Marlin refers to the project name of the particular branch of arduino firmware that the Taz uses. It is in turn derivative of the earlier sprinter firmware which is why you sometimes see Marlin/Sprinter as the name.
Cura will update the firmware automatically from compiled firmware if you like. If you want to build your own version, or edit settings you would want to change you can use the Arduino ide “integrated development environment” to compile and upload the individual c code based files. The process isn’t as scary as it sounds. You just make a folder, get all the files that make up the firmware and any support files, make your changes (usually configuration.h ) and hit the compile button. If you did it right, it will compile and you can then hit upload. If it errors, post them here and we can help fix them.
The one risk with firmware uploading is power loss. If you lose power before it finishes, it would brick the Rambo board. I recommend having the printer on a good stout ups if you want to edit firmware.
If you read through the whole 85 degree thread, you’ll see that 85 degrees is not the correct temp for the 5, just older models. The 5 at 110 is correct because of the PEI bed.
I think the purpose would be to “update” the preheat settings available through the LCD screen. As mentioned above the correct bed temp for ABS on the TAZ5 with the PEI is 110C. So no need to update the settings… 
Marlin is the name of the firmware which resides on the RAMBO controller board of the printer. To modify the firmware, parameters are changed in the “configuration.h” file and all the code needs to be re-compiled for a final hex to load on the printer. If you wanted to tweak the preloaded LCD settings, I believe they can be found in the CONFIGURATION.H file on line 761 “Preheat Constants”… or it might be somewhere else with settings for the menu…
Thanks, everyone! I really appreciate everyone’s helpfulness here.