Pick the fireweed blossoms from the stems, discarding any green pods or pieces, and clean off any dirt or insects.
Pour the boiling water over the blossoms in a heatproof container and steep for 10 to 15 minutes.
Strain the tea through a fine mesh strainer (or one lined with cheesecloth) into a saucepan or jam pot. Measure the tea, adding water if needed to reach the full amount, then stir in the lemon juice.
Bring the tea and lemon juice to a boil, then whisk in the powdered pectin until dissolved and boil hard for 1 full minute.
Add all the sugar at once and stir to dissolve. Return to a full rolling boil and boil hard for exactly 1 minute, then remove from heat and skim off any foam. (Do not add the sugar before or with the pectin, or the jelly won't set.)
Ladle hot jelly into prepared jars, leaving 1/4 inch headspace. Wipe rims clean, center lids, and apply bands fingertip-tight.
Process in a boiling water bath for 10 minutes, adjusting for altitude. Turn off the heat and let jars rest 5 minutes before removing. Cool undisturbed 12-24 hours before checking seals.