Inspired by a provocative (in a good way) comment by FumbleFingers (above), I did some research into the evolution of the definitions of combustible, flammable, and inflammable in Webster's dictionaries over the past 200 years. The entries for flammable are a bit spotty—absent from the 1806 dictionary, present in the next four editions (1828 through 1890), and then absent from the Collegiate series dictionaries until the Sixth Collegiate (1949). (I expect that flammable appears in the big New International (1909) and Second New International (1934) dictionaries, but those overlap with the Collegiate series, and I find the pared-down listings in the Collegiates a better guide to which words were in widespread, approved use, in the judgment of the Merriam-Webster authorities of the time.) The other two terms are listed in all of the dictionaries I consulted.
Here is a quick summary of how the wording of the relevant definitions has changed over the years:
A Compendious Dictionary of the English Language (1806):
Combustible, a. that will easily take fire or burn
Inflammable, a. easily set on fire, fiery, hot
An American Dictionary of the English Language (1828):
Combustible, a. That will take fire and burn; capable of catching fire; thus, wood and coal are combustible bodies.
Flammable, a. Capable of being enkindled into flame.
Inflammable, a. That may be set on fire; easily enkindled; susceptible of combustion; as inflammable oils or spirits.
An American Dictionary of the English Language (1847):
[no changes]
An American Dictionary of the English Language (1864):
Combustible, a, 1. Capable of taking fire and burning; inflammable.
Flammable, a, Capable of being enkindled into flame; inflammable. [Obs.]
Inflammable, a, Capable of being set on fire; easily enkindled; susceptible of combustion; as inflammable oils or spirits.
Webster's International Dictionary 1890):
Combustible, a, 1. Capable of taking fire and burning; apt to catch fire; inflammable.
Flammable, a, Inflammable. [Obs.]
Inflammable, a, 1. Capable of being easily set on fire; easily enkindled; combustible; as inflammable oils or spirits.
Webster's Collegiate Dictionary, First Edition (1898):
Combustible, a, 1. Capable of taking fire and burning; inflammable.
Inflammable, a, 1. Capable of being easily set on fire; combustible.
Webster's Collegiate Dictionary, Second Edition (1910):
[no changes]
Webster's Collegiate Dictionary, Third Edition (1916):
combustible, a, 1. Capable of combustion; inflammable.
inflammable [same as previous]
Webster's Collegiate Dictionary, Fourth Edition (1931):
[no changes]
Webster's Collegiate Dictionary, Fifth Edition (1936):
combustible, adj. Capable of combustion; inflammable; also, easily excited; irascible.
inflammable [same as previous]
Webster's New Collegiate Dictionary, Sixth Edition (1949):
combustible [same as previous]
flammable, adj. Capable of being easily ignited; inflammable;—preferred by many technical writers and publications to the older equivalent inflammable because of possible misinterpretation of the prefix in- as negative.
inflammable [same as previous]
Webster's New Collegiate Dictionary, Seventh Edition (1963):
combustible adj. 1 : Capable of combustion
flammable adj. capable of being easily ignited and of burning with extreme rapidity
inflammable adj. 1 : flammable
Webster's New Collegiate Dictionary, Eighth Edition (1973):
[no changes]
Webster's New Collegiate Dictionary, Ninth Edition (1983):
combustible [same as previous]
flammable adj. capable of being easily ignited and of burning quickly
inflammable [same as previous]
Merriam-Webster's Collegiate Dictionary, Tenth Edition (1993):
[no changes]
Merriam-Webster's Collegiate Dictionary, Eleventh Edition (2003):
[no changes]
...
I notice three especially interesting things about these evolving definitions. First, two very early definitions (those from 1828) support FumbleFingers' emphasis on the relatively easy and rapid burning behavior envisaged by inflammable: Whereas combustible is associated with wood and coal, inflammable is associated with oils and spirits. This important difference emerges despite previous definitions (in the 1806 dictionary) that offered no indication that such distinctions existed. Just as important, the "oils or spirits" example remains associated with inflammable through the most of the next century (all the way through the 1890 Webster's International dictionary).
Second, though the adverb easily persists in the definitions of inflammable of the middle period (from 1890 until 1963), the explicit inclusion of "combustible" as a definition of inflammable and of "inflammable" as a definition of combustible suggests something approaching interchangeability of the terms during this period, at least in general use.
Third, reemphasis of differences between the two terms begins in the Seventh Collegiate (1963), which clears the way for the new era of (relatively) stark contrast by removing "inflammable" as a definition of combustible for the first time since the 1847 dictionary, and by removing "combustible" as a definition of inflammable for the first time since the 1864 dictionary. The vivid addition to the definition of flammable in the Seventh Collegiate ("capable of being easily ignited and of burning with extreme rapidity") makes the revised definition so much more precise than the Sixth Collegiate's definitions of flammable ("Capable of being easily ignited; inflammable") and of inflammable ("Capable of being easily set on fire; combustible") that I can't help wondering whether the altered definitions of 1963 reflect a sudden dramatic shift between 1949 and 1963 in the way most people used those words, or whether Merriam-Webster's lexicographers were trying to establish a definition of flammable that was consistent with the terminology used by fire prevention authorities, regardless of how contemporaneous nonexpert writers understood the terms combustible, flammable, and inflammable.
Best Answer
Just and simply have some overlapping meanings, but definitely do not overlap in all meanings. In your example, it is a case where they have essentially the same meaning.
However, just (the adverb) can also mean "only now". For example:
Simply cannot be used in this instance.
(Note: I assume that we are only talking about just as an adverb; just can be used as an adjective too, meaning "right" or "fair". I don't think you are concerned about that one, because it is clearly totally different in usage.)