Hey! I’ve recently been delving into Firebrand as well and figured I could share some of the knowledge I’ve learned with you. If somebody is more knowledgable, I’m sure they’ll correct me.
I assume you’re referring to Mantra, which is a type of skill Firebrands (and Mesmers) have at their disposal. For Firebrand, think of it as a skill with 3 individual uses (or charges as they’re referred as) before it enters a cooldown state, the time it takes until they are all able to be used again.
https://wiki.guildwars2.com/wiki/Mantra
Something to consider about Firebrand’s mantra usage is that each charge is functionally the same, but typically the last charge has a bit more bang in the effects it brings. The first two charges also has its own refresh period where you will get a charge back, sort of like a cooldown state, but the third, more powerful charge will enter a longer cooldown state. If you are still in combat when the cooldown is over, you’ll have to “enable” the mantra for use again, it’s usually a couple seconds of channeling. If you are not in combat when the cooldown is over, your mantra will skip the channel and you’ll have all three charges again.
Mantra of Solace is a healing focused mantra, it will give a burst of healing with each use to you and nearby party members. Mantra of Potence is similar in function, but it will apply the quickness boon instead of healing.
Presuming you’re playing something close to Condi Firebrand, the rotation can feel overwhelming but focus on the big things first—you’ll rarely need to enter F2s or F3s tomes so forget about those 10 skills for now. The first tome is your primary damage and you’ll ideally cast skills 5, 4 and 2 in that order and typically stow the book away while the skills recharge.
You’ll swap to your axe/torch and always use the 2 skill from it as it comes off cooldown, use any procs from your 4 skill and use 5 when 2 isn’t available. Auto attacking in between and be sure to finish the chain when you begin it.
You’ll return to your F1 tome when it’s generated the charges to use it’s 5, 4, and 2 again. Plug in any utility as needed during any of this in tome or axe/torch.
You can take “Hold the Line!” if you would like and you may run into situations where it could be helpful, but ultimately there is better skills for damage purposes or utility.
Hope this helps!